Method and base station for transmitting downstream link data, and method and user device for receiving downstream link data

ABSTRACT

The present invention relates to a method and apparatus which transmit/receive at least one demodulation reference signal by using a CDM group and/or a transmission rank of a user device that have been used to transmit the at least one demodulation reference signal for the user device, an OCC that has been used to spread the demodulation reference signal, etc. Also, the present invention relates to a method and apparatus which change an antenna port for transmitting the demodulation reference signal by using NDI for a disabled transmission block.

This Application is a 35 U.S.C. §371 National Stage Entry ofInternational Application No. PCT/KR2011/000072, filed Jan. 7, 2011 andclaims the benefit of U.S. Provisional Application Nos. 61/296,389,filed Jan. 19, 2010, 61/320,331, filed Apr. 2, 2010, and KoreanApplication No: 10-2011-0000470, filed Jan. 4, 2011, all of which areincorporated by reference in their entirety herein.

FIELD OF THE INVENTION

The present invention relates to a wireless communication system, andmore particularly to a method and apparatus for transmitting/receivingdownlink data, and a method and apparatus for indicating and/ordetecting a reference signal for demodulation of the downlink data.

BACKGROUND ART

Recently, in order to maximize the performance of the wirelesscommunication system and the communication capacity, Multiple InputMultiple Output (MIMO) systems have been drawing a great deal ofattention. The MIMO technology corresponds to an evolved version of theconventional communication technology using a single transmissionantenna and a single reception antenna, and the MIMO technology appliesmultiple transmission antennas and multiple reception antennas, so as toenhance the transmitted and/or received (or transceived) datatransmission efficiency. The MIMO technology applies the technology ofreceiving a plurality of segmented data fragments being transmittedthrough multiple antennas and completing the received message bygrouping the collected data fragments, instead of relying on a singleantenna path, in order to receive a single full message. As s result,the data transmission rate may be enhanced within a predetermined range,or a system range may be increased with respect to a specific datatransmission rate.

The MIMO technology may include transmit diversity, spatialmultiplexing, and beamforming. Herein, transmit diversity corresponds toa technology of transmitting the same type of data through multipletransmission antennas, so as to enhance the transmission reliability.Spatial multiplexing corresponds to a technology of having differenttypes of data being transmitted at the same time through multipleantennas, thereby being capable of transmitting data at a fasttransmission rate, without having to increase the system bandwidth.Moreover, beamforming is used for increasing an SINR (Signal toInterference plus Noise Ratio) of a signal by adding a weight respectiveto a channel state (or status) in a multiple antennas system. At thispoint, the weight may be expressed as a weight vector or a weightmatrix, and this may also be referred to as a precoding vector or aprecoding matrix.

Spatial multiplexing may include spatial multiplexing respective to asingle user and spatial multiplexing respective to a plurality of users.Accordingly, spatial multiplexing may also be referred to as a singleuser MIMO (SU-MIMO), and spatial multiplexing respective to a pluralityof users may also be referred to as SDMA (Spatial Division MultipleAccess) or Multi User MIMO (MU-MIMO).

The capacity of a MIMO channel increases in proportion to the number ofantennas. The MIMO channel may be separated (or divided) to independentchannels. When the number of transmission antennas is referred to asN_(t), and when the number of reception antennas is referred to asN_(r), the number of independent channels N, becomes N_(t)=min{N_(t),N_(r)}. Each of the independent channels may be referred to as a spatiallayer. As a non-zero eigenvalue of a MIMO channel matrix, a rank may bedefined by a number of spatial streams that can be multiplexed.

As shown in the example of a single user MIMO shown in FIG. 1, thesingle user MIMO corresponds to a structure wherein multiple datastreams, each being different from one another, transmitted from thebase station are all transmitted to a single user. In case of the singleuser MIMO, a MIMO channel consists of one transmitter and one receiver.In case of the single user MIMO, one user may receive all of thetransmitted signals. Therefore, in case of the single user MIMO, onlythe data respective to a single user are scheduled to the sametime/frequency domain(s). Conversely, as shown in the example of amultiple-user MIMO shown in FIG. 2, the multiple-user MIMO respectivelytransmits the multiple data streams, each being different from oneanother and being transmitted from the base station, to the plurality ofusers. In case of the multiple-user MIMO, one transmitter and multiplereceivers collectively configure the MIMO channel. Therefore, in case ofthe multiple-user MIMO, the data respective to the plurality of usersmay be collectively scheduled to the same time/frequency domain(s).

User equipment (UE) operation modes are classified into an SU-MIMO modeand an MU-MIMO mode in terms of spatial multiplexing. The UE may becomprised of the SU-MIMO mode or the MU-MIMO mode through signaling froma base station (BS). In this case, the UE is designed to semi-staticallyswitch a transmission mode between the SU-MIMO mode and the MU-MIMOmode. In order to perform transmission mode switching between theSU-MIMO mode and the MU-MIMO mode, the UE requires a long switchingtime, resulting in deterioration of overall system throughput.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

Accordingly, the present invention is directed to a downlink datatransmission method and a base station (BS), and a downlink receptionmethod and a user equipment (UE), that substantially obviate one or moreproblems due to limitations and disadvantages of the related art. Anobject of the present invention is to provide a method and apparatus forreceiving not only data transmitted by SU-MIMO transmission but alsoanother data transmitted by MU-MIMO transmission, irrespective of UEmode switching.

Another object of the present invention is to provide a method andapparatus for transmitting, by a user equipment (UE), necessaryinformation so as to efficiently demodulate user data transmitted to theuser equipment (UE).

Another object of the present invention is to provide a method andapparatus for transmitting necessary information for user datamodulation while simultaneously minimizing signaling overhead.

Another object of the present invention is to provide a method andapparatus for demodulating SU-MIMO transmission data and MU-MIMOtransmission data on the basis of necessary information for user datademodulation.

It is to be understood that technical objects to be achieved by thepresent invention are not limited to the aforementioned technicalobjects and other technical objects which are not mentioned will beapparent from the following description to the person with an ordinaryskill in the art to which the present invention pertains.

Technical Solution

The object of the present invention can be achieved by providing amethod for transmitting downlink data to a user equipment (UE) by a basestation (BS) in a wireless communication system, the method including:transmitting at least one demodulation reference signal (DMRS) fordemodulating the downlink data to the user equipment (UE) on the basisof a predetermined mapping type from among a plurality of mapping types,wherein each mapping type defines a code division multiplexing (CDM)group for DMRS transmission and an orthogonal cover code (OCC) for DMRSspreading according to individual antenna ports; and transmitting typeinformation indicating the predetermined mapping type and rankinformation indicating the number of transmission layers of the downlinkdata.

In another aspect of the present invention, a method for receivingdownlink data from a base station (BS) by a user equipment (UE) in awireless communication system includes receiving information indicatinga predetermined mapping type from among a plurality of mapping types andrank information indicating the number of transmission layers of thedownlink data, wherein each mapping type defines a code divisionmultiplexing (CDM) group for DMRS transmission and an orthogonal covercode (OCC) for DMRS spreading according to individual antenna ports;detecting at least one demodulation reference signal (DMRS) transmittedfor demodulating the downlink data on the basis of the predeterminedmapping type and the rank information; and receiving each transmissionlayer of the downlink data using the at least one DMRS.

In another aspect of the present invention, a base station (BS) fortransmitting downlink data to a user equipment (UE) in a wirelesscommunication system includes a transmitter configured to transmit thedownlink data to the user equipment (UE); and a processor forcontrolling the transmitter to transmit at least one demodulationreference signal (DMRS) demodulating the downlink data to the userequipment (UE) on the basis of a predetermined mapping type from among aplurality of mapping types, wherein each mapping type defines a codedivision multiplexing (CDM) group for DMRS transmission and anorthogonal cover code (OCC) for DMRS spreading according to individualantenna ports, as well as to transmit type information indicating thepredetermined mapping type and rank information indicating the number oftransmission layers of the downlink data.

In another aspect of the present invention, a user equipment (UE) forreceiving downlink data from a base station (BS) in a wirelesscommunication system includes a receiver; and a processor configured tocontrol the receiver, wherein the receiver receives informationindicating a predetermined mapping type from among a plurality ofmapping types and rank information indicating the number of transmissionlayers of the downlink data, and transmits the received information tothe processor, wherein each mapping type defines a code divisionmultiplexing (CDM) group for DMRS transmission and an orthogonal covercode (OCC) for DMRS spreading according to individual antenna ports,wherein the processor controls the receiver to detect at least onedemodulation reference signal (DMRS) transmitted for demodulating thedownlink data on the basis of the predetermined mapping type and therank information, and controls the receiver to receive each transmissionlayer of the downlink data using the at least one DMRS.

The base station (BS) transmits information indicating a start DMRS fromamong the at least one DMRS to the user equipment (UE).

In another aspect of the present invention, a method for transmittingdownlink data to a user equipment (UE) by a base station (BS) in awireless communication system includes multiplexing at least onedemodulation reference signal (DMRS) used for demodulating the downlinkdata to a predetermined code division multiplexing (CDM) group; andtransmitting the multiplexed DMRS through the predetermined CDM group,wherein rank information for indicating the number of transmissionlayers of the downlink data is transmitted and CDM group indicationinformation for indicating the predetermined CDM group is transmitted.

In another aspect of the present invention, a method for receivingdownlink data from a base station (BS) by a user equipment (UE) in awireless communication system includes receiving rank information forindicating the number of transmission layers of the downlink data andcode division multiplexing (CDM) group indication information forindicating a predetermined CDM group; detecting at least onedemodulation reference signal (DMRS) transmitted through thepredetermined CDM group for demodulating the downlink data, using therank information and the CDM group indication information; and receivingeach transmission layer of the downlink data using the detected DMRS.

In another aspect of the present invention, a base station (BS) fortransmitting downlink data to a user equipment (UE) in a wirelesscommunication system includes a transmitter configured to transmit thedownlink data to the user equipment (UE); and a processor configured tocontrol the transmitter, wherein the processor controls the transmitterto multiplex at least one demodulation reference signal (DMRS) used fordemodulating the downlink data to a predetermined code divisionmultiplexing (CDM) group, and controls the transmitter to transmit themultiplexed DMRS through the predetermined CDM group, and controls thetransmitter to transmit rank information for indicating the number oftransmission layers of the downlink data and CDM group indicationinformation for indicating the predetermined CDM group.

In another aspect of the present invention, a user equipment (UE) forreceiving downlink data from a base station (BS) in a wirelesscommunication system includes a receiver; and a processor configured tocontrol the receiver which receives rank information for indicating thenumber of transmission layers of the downlink data and code divisionmultiplexing (CDM) group indication information for indicating apredetermined CDM group, and transfers the received information to theprocessor, wherein the processor controls the receiver to detect atleast one demodulation reference signal (DMRS) transmitted through thepredetermined CDM group for demodulating the downlink data using therank information and the CDM group indication information, and alsocontrols the receiver to receive each transmission layer of the downlinkdata using the detected DMRS.

The at least one DMRS is spread per DMRS by at least one orthogonalcover code (OCC), the spread DMRS is multiplexed to the predeterminedCDM group, and the base station (BS) can transmit start OCC indicationinformation indicating a start OCC from among the at least one OCC tothe user equipment (UE).

In another aspect of the present invention, a method for transmitting asingle transport block (TB) acting as at least one transmission layer toa user equipment (UE) by a base station (BS) capable of transmitting aplurality of transport blocks (TBs) in a wireless communication systemincludes transmitting the at least one transmission layer to the userequipment (UE) either through at least one antenna port contained in afirst antenna port group or through at least one antenna port containedin a second antenna port group; if the at least one transmission layeris transmitted through the first antenna port group, transmittingdownlink control information in which a new data indicator (NDI) of eachtransport block (TB) other than the single transport block (TB) is setto a first value so as to indicate the first antenna port group, to theuser equipment (UE); and if the at least one transmission layer istransmitted through the second antenna port group, transmitting downlinkcontrol information in which a new data indicator (NDI) of eachtransport block (TB) other than the single transport block (TB) is setto a second value so as to indicate the second antenna port group, tothe user equipment (UE).

In another aspect of the present invention, a method for receiving asingle transport block (TB) acting as at least one transmission layerfrom a base station (BS) by a user equipment (UE) capable oftransmitting a plurality of transport blocks (TBs) in a wirelesscommunication system includes receiving downlink control informationfrom the base station (BS); and receiving the at least one transmissionlayer transmitted either through a first antenna port group or through afirst antenna port group on the basis of a new data indicator (NDI) foreach TB contained in the downlink control information, wherein thedownlink control information includes control information in which, ifthe at least one transmission layer is transmitted through the firstantenna port group, a new data indicator (NDI) of each transport block(TB) other than the single transport block (TB) is set to a first valueso as to indicate the first antenna port group; and wherein the downlinkcontrol information includes control information in which, if the atleast one transmission layer is transmitted through the second antennaport group, a new data indicator (NDI) of each transport block (TB)other than the single transport block (TB) is set to a second value soas to indicate the second antenna port group.

In another aspect of the present invention, a base station (BS) fortransmitting a single transport block (TB) acting as at least onetransmission layer to a user equipment (UE) so as to transmit aplurality of transport blocks (TBs) in a wireless communication systemincludes a transmitter configured to transmit the downlink data to theuser equipment (UE); and a processor configured to control thetransmitter, wherein the processor controls the transmitter to transmitthe at least one transmission layer to the user equipment (UE) eitherthrough at least one antenna port contained in a first antenna portgroup or through at least one antenna port contained in a second antennaport group; the processor controls the transmitter such that, if the atleast one transmission layer is transmitted through the first antennaport group, the transmitter transmits downlink control information inwhich a new data indicator (NDI) of each transport block (TB) other thanthe single transport block (TB) is set to a first value so as toindicate the first antenna port group, to the user equipment (UE); andthe processor controls the transmitter such that, if the at least onetransmission layer is transmitted through the second antenna port group,the transmitter transmits downlink control information in which a newdata indicator (NDI) of each transport block (TB) other than the singletransport block (TB) is set to a second value so as to indicate thesecond antenna port group, to the user equipment (UE).

In another aspect of the present invention, a user equipment (UE) forreceiving a single transport block (TB) acting as at least onetransmission layer from a base station (BS) so as to transmit aplurality of transport blocks (TBs) in a wireless communication systemincludes a receiver; and a processor configured to control the receiver,wherein the receiver receives downlink control information from the basestation (BS) and transmits the received information to the processor,the processor controls the receiver to receive the at least onetransmission layer transmitted either through a first antenna port groupor through a first antenna port group on the basis of a new dataindicator (NDI) for each TB contained in the downlink controlinformation. The downlink control information includes controlinformation in which, if the at least one transmission layer istransmitted through the first antenna port group, a new data indicator(NDI) of each transport block (TB) other than the single transport block(TB) is set to a first value so as to indicate the first antenna portgroup; and includes another control information in which, if the atleast one transmission layer is transmitted through the second antennaport group, a new data indicator (NDI) of each transport block (TB)other than the single transport block (TB) is set to a second value soas to indicate the second antenna port group.

The aforementioned technical solutions are only a part of theembodiments of the present invention, and various modifications to whichtechnical features of the present invention are applied could beunderstood by the person with ordinary skill in the art to which thepresent invention pertains, based on the following detailed descriptionof the present invention.

Effects of the Invention

As is apparent from the above description, exemplary embodiments of thepresent invention have the following effects.

In accordance with the embodiments of the present invention, although aMIMO transmission mode is not changed, a user equipment (UE) can receiveSU-MIMO transmission data and MU-MIMO transmission data. As a result, atime required for switching a transmission/reception (Tx/Rx) mode of theuser equipment (UE) is reduced, resulting in an increase in overallsystem throughput.

In accordance with the present invention, the user equipment (UE) canefficiently demodulate user data transmitted to the user equipment (UE).

The base station (BS) according to the present invention can transmitnecessary information for user data modulation while simultaneouslyminimizing signaling overhead.

The user equipment (UE) according to the present invention candemodulate SU-MIMO transmission data and MU-MIMO transmission data onthe basis of necessary information for user data demodulation.

It will be appreciated by persons skilled in the art that the effectsthat could be achieved with the present invention are not limited towhat has been particularly described hereinabove and other advantages ofthe present invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 is a conceptual diagram illustrating an example of SU-MIMO.

FIG. 2 is a conceptual diagram illustrating an example of multi-userMIMO;

FIG. 3 is a block diagram illustrating constituent elements of a userequipment (UE) and a base station (BS) according to the presentinvention;

FIG. 4 is a conceptual diagram illustrating a process for processingsignals of a transmitter for use in a UE and a BS;

FIG. 5 is a diagram illustrating a radio frame structure for use in awireless communication system;

FIG. 6 is an exemplary structural diagram illustrating DL/UL slotstructures for use in a wireless communication system;

FIG. 7 is a diagram illustrating an example of an downlink subframestructure for use in a wireless communication system;

FIG. 8 is a diagram illustrating an example of an uplink subframestructure for use in a wireless communication system;

FIG. 9 is a conceptual diagram illustrating DRS transmission;

FIG. 10 is a diagram illustrating an example of DRS pattern for use inan LTE system;

FIG. 11 is a diagram illustrating DMRS patterns of antenna ports #7˜#10;

FIG. 12 shows resource elements (REs) obtained when DMRSs aremultiplexed using an OCC having a length of 2;

FIG. 13 shows resource elements (REs) obtained when DMRSs aremultiplexed using an OCC having a length of 4; and

FIG. 14 is a flowchart illustrating a method for processing data using aDMRS.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. It is to beunderstood that the detailed description, which will be disclosed alongwith the accompanying drawings, is intended to describe the exemplaryembodiments of the present invention, and is not intended to describe aunique embodiment with which the present invention can be carried out.The following detailed description includes detailed matters to providefull understanding of the present invention. However, it will beapparent to those skilled in the art that the present invention can becarried out without the detailed matters.

Techniques, apparatus and systems described herein can be used invarious wireless access technologies such as Code Division MultipleAccess (CDMA), Frequency Division Multiple Access (FDMA), Time DivisionMultiple Access (TDMA), Orthogonal Frequency Division Multiple Access(OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA),etc. The CDMA may be implemented with a radio technology such asUniversal Terrestrial Radio Access (UTRA) or CDMA2000. The TDMA may beimplemented with a radio technology such as Global System for Mobilecommunications (GSM)/General Packet Radio Service (GPRS)/Enhanced DataRates for GSM Evolution (EDGE). The OFDMA may be implemented with aradio technology such as institute of electrical and electronicsengineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20,Evolved-UTRA (E-UTRA) etc. The UTRA is a part of a Universal MobileTelecommunication System (UMTS). 3rd Generation Partnership Project(3GPP) Long Term Evolution (LTE) is a part of an Evolved-UMTS (E-UMTS)using the E-UTRA. The 3GPP LTE employs the OFDMA in downlink (DL) andemploys the SC-FDMA in uplink (UL). LTE-Advance (LTE-A) is an evolutionof the 3GPP LTE. For clarity, this application focuses on the 3GPPLTE/LTE-A. However, technical features of the present invention are notlimited thereto. For example, although the following description will bemade based on a mobile communication system corresponding to a 3GPPLTE/LTE-A system, the following description can be applied to othermobile communication systems except unique features of the 3GPPLTE/LTE-A system.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

In the present invention, a User Equipment (UE) denotes a mobile orfixed type user terminal. Examples of the UE include various equipmentsthat transmit and receive user data and/or various kinds of controlinformation to and from a base station. The UE may be referred to as, aTerminal Equipment (TE), a Mobile Station (MS), a Mobile Terminal (MT),a User Terminal (UT), a Subscriber Station (SS), a wireless device, aPersonal Digital Assistant (PDA), a wireless modem, or a handhelddevice. Also, in the present invention, a Base Station (BS) means afixed station that performs communication with a user equipment and/oranother base station, and exchanges various kinds of data and controlinformation with the user equipment and another base station. The basestation may be referred to another terminology such as an Evolved-NodeB(eNB), a Base Transceiver System (BTS), and an Access Point (AP).

In the present invention, PDCCH(Physical Downlink ControlCHannel)/PCFICH(Physical Control Format IndicatorCHannel)/PHICH(Physical Hybrid-ARQ Indicator CHannel)/PDSCH(PhysicalDownlink Shared CHannel)/DRS(Dedicate Reference Signal)/CRS(CommonReference Signal)/DMRS(DeModulation Reference Signal)/CSI-RS(ChannelState Information Reference Signal) resource element (RE) represents anRE assigned to or available forPDCCH/PCFICH/PHICH/PDSCH/DRS/CRS/DMRS/CSI-RS. In particular, a resourceelement (RE) carrying a reference signal shall be named RS RE and aresource element (RE) carrying control information or data shall benamed data RE.

Hereinafter, symbol/carrier/subcarrier to which DRS/CRS/DMRS/CSI-RS isassigned is referred to as DRS/CRS/DMRS/CSI-RSsymbol/carrier/subcarrier. For example, a symbol to which DRS isassigned is referred to as a DRS symbol, and a subcarrier to which DRSis assigned is referred to as a DRS subcarrier. Besides, a symbol towhich user data (for example, PDSCH data, PDCCH data, and etc.) isassigned is referred to as data symbol, and a subcarrier to which userdata is assigned is referred to as data subcarrier.

Meanwhile, in the present invention, if a specific signal is allocatedto frame/subframe/slot/symbol/carrier/subcarrier, it means that thespecific signal is transmitted through the correspondingcarrier/subcarrier during a period/timing of the correspondingframe/subframe/slot/symbol.

Hereinafter, if the special signal is allocated tosuperframe/frame/subframe/symbol/carrier/subcarrier, it means that thespecial signal is transmitted through the correspondingcarrier/subcarrier during a period/timing of the correspondingsuperframe/frame/subframe/symbol.

FIG. 3 illustrates a block view showing elements configuring a userequipment and base station performing the method according to thepresent invention.

The user equipment operates as a transmitting device in an uplink andoperates as a receiving device in a downlink. The base station operatesas a receiving device in an uplink and operates as a transmitting devicein a downlink.

Each of the user equipment and the base station includes antenna(s) (500a, 500 b) that can receive information and/or data, signals, messages,and so on, a Transmitter (100 a, 100 b) transmitting messages bycontrolling the antenna(s), a Receiver (300 a, 300 b) receiving messagesby controlling the antenna(s), and a memory (200 a, 200 b) storingvarious information related to communication within the wirelesscommunication system. Also, each of the user equipment and the basestation includes a processor (400 a, 400 b), which is configured toperform the present invention by controlling the elements included inthe user equipment and the base station, such as the transmitter and thereceiver, and the memory, and so on. The transmitter (100 a), thereceiver (300 a), the memory (200 a), and the processor (400 a) includedin the user equipment may each be implemented as independent elements byusing separate chips, or a combination of at least two or more elementsmay be implemented by using a single chip. Similarly, the transmitter(100 b), the receiver (300 b), the memory (200 b), and the processor(400 b) included in the base station may each be implemented asindependent elements by using separate chips, or a combination of atleast two or more elements may be implemented by using a single chip.The transmitter and receiver may also be combined so as to beimplemented as a single transceiver within the user equipment or thebase station.

The antenna (500 a, 500 b) performs the function of transmitting asignal, which is generated from the transmitter (100 a, 100 b), to anoutside target, or the antenna(s) (500 a, 500 b) performs the functionsof receiving a radio signal from an outside source and delivering thereceived radio signal to the receiver (300 a, 300 b). In case of atransceiving module supporting the Multi-Input Multi-Output (MIMO)function, which transmits and receives data by using multiple antennas,the transceiving module may be connected to 2 or more antennas.

The processor (400 a, 400 b) generally controls the overall operationsof each module within the user equipment or the base station. Mostparticularly, the processor (400 a, 400 b) may perform various controlfunctions for performing the present invention, MAC (Medium AccessControl) frame variable control functions respective to servicecharacteristics and frequency environments, power saving mode functionsfor controlling idle mode operations, Hand Over functions, certificationand encryption functions, and so on. The processor (400 a, 400 b) mayalso be referred to as a controller, a microcontroller, amicroprocessor, a microcomputer, and so on. Meanwhile, the processor(400 a, 400 b) may be implemented in the form of hardware or firmware,or software, or in a combination of at least two or more of hardware,firmware, and software. In case of implementing the embodiments of thepresent invention by using hardware, ASICs (Application SpecificIntegrated Circuits) or DSPs (Digital Signal Processors), DSPDs (DigitalSignal Processing Devices), PLDs (Programmable Logic Devices), FPGAs(Field Programmable Gate Arrays), and so on, which are configured toperform the present invention, may be provided in the processor (400 a,400 b). Meanwhile, in case of implementing the embodiments of thepresent invention by using firmware or software, the firmware orsoftware may be configured to include a module, procedure, or functionperforming the above-described functions or operations, and the firmwareor software, which is configured to perform the present invention may beprovided in the processor (400 a, 400 b), or may be stored in the memory(200 a, 200 b) so as to be operated by the processor (400 a, 400 b).

The transmitter (100 a, 100 b) performs coding and modulation on signalsand/or data that are to be scheduled by the processor (400 a, 400 b) orby a scheduler being connected to the processor, so as to be transmittedto an outside target and, then, transmits the processed signals and/ordata to the antenna (500 a, 500 b). For example, the transmitter (100 a,100 b) converts a data sequence that is to be transmitted to K number oflayers by performing demultiplexing, channel encoding, and modulationprocesses. The K number of layers passes through a transmissionprocessor included in the transmitter, so as to be transmitted throughthe transmitting antenna (500 a, 500 b). The transmitter (100 a, 100 b)and the receiver (300 a, 300 b) of the user equipment (12) and the basestation (11) may each be differently configured depending upon theprocedures for processing the transmission signal and the receptionsignal.

FIG. 4 is a block diagram of an exemplary transmitter in each of the UEand the BS. Operations of the transmitters 100 a and 100 b will bedescribed below in more detail with reference to FIG. 4.

Referring to FIG. 4, each of the transmitters 100 a and 100 b includescramblers 301, modulation mappers 302, a layer mapper 303, a precoder304, RE mappers 305, Orthogonal Frequency Division Multiplexing/SingleCarrier Frequency Division Multiplexing (OFDM/SC-FDM) signal generators306.

The transmitters 100 a and 100 b may transmit more than one codeword.The scramblers 301 scramble the coded bits of each codeword, fortransmission on a physical channel. A codeword may be referred to as adata stream and is equivalent to a data block from the MAC layer. Thedata block from the MAC layer is referred to as a transport block.

The modulation mappers 302 modulate the scrambled bits, thus producingcomplex modulation symbols. The modulation mappers 302 modulate thescrambled bits to complex modulation symbols representing positions on asignal constellation in a predetermined modulation scheme. Themodulation scheme may be, but not limited to, any of m-Phase ShiftKeying (m-PSK) and m-Quadrature Amplitude Modulation (m-QAM).

The layer mapper 303 maps the complex modulation symbols to one orseveral transmission layers.

The precoder 304 may precode the complex modulation symbols on eachlayer, for transmission through the antenna ports. More specifically,the precoder 304 generates antenna-specific symbols by processing thecomplex modulation symbols for multiple transmission antennas 500-1 to500-N_(t) in a MIMO scheme, and distributes the antenna-specific symbolsto the RE mappers 305. That is, the precoder 304 maps the transmissionlayers to the antenna ports. The precoder 304 may multiply an output xof the layer mapper 303 by an N_(t)×M_(t) precoding matrix W and outputthe resulting product in the form of an N_(t)×M_(F) matrix z.

The RE mappers 305 map/allocate the complex modulation symbols for therespective antenna ports to REs. The RE mappers 305 may allocate thecomplex modulation symbols for the respective antenna ports toappropriate subcarriers, and may multiplex them according to users.

The OFDM/SC-FDM signal generators 306 modulate the complex modulationsymbols for the respective antenna ports, that is, the antenna-specificsymbols through OFDM or SC-FDM modulation, thereby producing a complextime-domain OFDM or SC-FDM symbol signal. The OFDM/SC-FDM signalgenerators 306 may perform Inverse Fast Fourier Transform (IFFT) on theantenna-specific symbols and insert a Cyclic Prefix (CP) into theresulting IFFT time-domain symbol. The OFDM symbol is transmittedthrough the transmission antennas 500-1 to 500-N_(t) to a receiver afterdigital-to-analog conversion, frequency upconversion, etc. TheOFDM/SC-FDM signal generators 306 may include an IFFT module, a CPinserter, a Digital-to-Analog Converter (DAC), a frequency upconverter,etc.

If the transmitters 100 a and 100 b adopt SC-FDMA for transmitting acodeword, the transmitters 100 a and 100 b include an FFT processor (notshown). The FFT processor performs FFT on the complex modulation symbolsfor each antenna and outputs the FFT symbol to the RE mappers 305.

The receivers 300 a and 300 b operate in the reverse order to theoperation of the transmitters 100 a and 100 b. The receivers 300 a and300 b decode and demodulate radio signals received through the antennas500 a and 500 b from the outside and transfer the demodulated signals tothe processors 400 a and 400 b. The antenna 500 a or 500 b connected toeach of the receivers 300 a and 300 b may include N_(r) receptionantennas. A signal received through each reception antenna isdownconverted to a baseband signal and then recovered to the originaldata stream transmitted by the transmitter 100 a or 100 b throughmultiplexing and MIMO demodulation. Each of the receivers 300 a and 300b may include a signal recoverer for downconverting a received signal toa baseband signal, a multiplexer for multiplexing received signals, anda channel demodulator for demodulating the multiplexed signal stream toa codeword. The signal recoverer, the multiplexer, and the channeldemodulator may be configured into an integrated module for performingtheir functions or independent modules. To be more specific, the signalrecoverer may include an Analog-to-Digital Converter (ADC) forconverting an analog signal to a digital signal, a CP remover forremoving a CP from the digital signal, an FFT module for generating afrequency-domain symbol by performing FFT on the CP-removed signal, andan RE demapper/equalizer for recovering antenna-specific symbols fromthe frequency-domain symbol. The multiplexer recovers transmissionlayers from the antenna-specific symbols and the channel demodulatorrecovers the codeword transmitted by the transmitter from thetransmission layers.

If the receivers 300 a and 300 b receive signals transmitted by SC-FDMA,each of the receivers 300 a and 300 b further includes an IFFT module.The IFFT module IFFT-processes the antenna-specific symbols recovered bythe RE demapper and outputs the IFFT symbol to the multiplexer.

While it has been described in FIGS. 3 and 4 that each of thetransmitters 100 a and 100 b includes the scramblers 301, the modulationmappers 302, the layer mapper 303, the precoder 304, the RE mappers 305,and the OFDM/SC-FDM signal generators 306, it may be furthercontemplated that the scramblers 301, the modulation mappers 302, thelayer mapper 303, the precoder 304, the RE mappers 305, and theOFDM/SC-FDM signal generators 306 are incorporated into each of theprocessors 400 a and 400 b of the transmitters 100 a and 100 b.Likewise, while it has been described in FIGS. 3 and 4 that each of thereceivers 300 a and 300 b includes the signal recoverer, themultiplexer, and the channel demodulator, it may be further contemplatedthat the signal recoverer, the multiplexer, and the channel demodulatorare incorporated into each of the processors 400 a and 400 b of thereceivers 300 a and 300 b. For the convenience's sake of description,the following description will be given with the appreciation that thescramblers 301, the modulation mappers 302, the layer mapper 303, theprecoder 304, the RE mappers 305, and the OFDM/SC-FDM signal generators306 are included in the transmitters 100 a and 100 b configuredseparately from the processors 400 a and 400 b that controls theiroperations, and the signal recoverer, the multiplexer, and the channeldemodulator are included in the receivers 300 a and 300 b configuredseparately from the processors 400 a and 400 b that controls theiroperations. However, it is to be noted that even though the scramblers301, the modulation mappers 302, the layer mapper 303, the precoder 304,the RE mappers 305, and the OFDM/SC-FDM signal generators 306 areincluded in the processors 400 a and 400 b or the signal recoverer, themultiplexer, and the channel demodulator are included in the processors400 a and 400 b, embodiments of the present invention are applicable inthe same manner.

The memories 200 a and 200 b may store programs required for signalprocessing and controlling of the processors 400 a and 400 b andtemporarily store input and output information. The memories 200 a and200 b may store codebooks according to the exemplary embodiment of thepresent invention to be described later. The memories 200 a and 200 bmay store predefined codebooks with respect to each rank. Each of thememories 200 a and 200 b may be implemented into a flash memory-typestorage medium, a hard disc-type storage medium, a multimedia cardmicro-type storage medium, a card-type memory (e.g. a Secure Digital(SD) or eXtreme Digital (XS) memory), a Random Access Memory (RAM), aRead-Only Memory (ROM), an Electrically Erasable Programmable Read-OnlyMemory (EEPROM), a Programmable Read-Only Memory (PROM), a magneticmemory, a magnetic disc, or an optical disk.

FIG. 5 illustrates an exemplary structure of a radio frame in a wirelesscommunication system. Specifically, the radio frame is a 3GPP LTE/LTE-Aradio frame. The radio frame structure is applicable to a FrequencyDivision Duplex (FDD) mode, a half FDD (H-FDD) mode, and a Time DivisionDuplex (TDD) mode.

Referring to FIG. 5, a 3GPP LTE/LTE-A radio frame is 10 ms (307, 200T_(s)) in duration. The radio subframe is divided into 10 equally-sizedsubframes, each subframe being 1 ms long. T_(s) represents a samplingtime and is given as T_(s)=1/(2048×15 kHz). Each subframe is furtherdivided into two slots, each of 0.5 ms in duration. 20 slots aresequentially numbered from 0 to 19. A time interval in which onesubframe is transmitted is defined as a Transmission Time Interval(TTI).

FIG. 6 illustrates an exemplary structure of a DownLink/UpLink (DL/UL)slot in the wireless communication system. Specifically, FIG. 6illustrates the structure of a resource grid in the 3GPP LTE/LTE-Asystem.

Referring to FIG. 6, a slot includes a plurality of OFDM symbols in thetime domain by a plurality of Resource Blocks (RBs) in the frequencydomain. An OFDM symbol may refer to one symbol duration. An RB includesa plurality of subcarriers in the frequency domain. An OFDM symbol maybe called an OFDM symbol, an SC-FDM symbol, etc. according to a multipleaccess scheme. The number of OFDM symbols per slot may vary depending ona channel bandwidth and a CP length. For instance, one slot includes 7OFDM symbols in case of a normal CP, whereas one slot includes 6 OFDMsymbols in case of an extended CP. While a subframe is shown in FIG. 6as having a slot with 7 OFDM symbols for illustrative purposes,embodiments of the present invention are also applicable to subframeswith any other number of OFDM symbols. A resource including one OFDMsymbol by one subcarrier is referred to as a Reference Element (RE) or atone.

Referring to FIG. 6, a signal transmitted in each slot may be describedby a resource grid including N^(DL/UL) _(RB)N^(RB) _(sc) subcarriers andN^(DL/UL) _(symb) OFDM or SC-FDM symbols. N^(DL) _(RB) represents thenumber of RBs in a DL slot and N^(UL) _(RB) represents the number of RBsin a UL slot. N^(DL) _(symb) represents the number of OFDM or SC-FDMAsymbols in the DL slot and N^(UL) _(symb) represents the number of OFDMor SC-FDMA symbols in the UL slot. N^(RB) _(sc) represents the number ofsubcarriers in one RB.

In other words, a Physical Resource Block (PRB) is defined as N^(DL/UL)_(symb) consecutive OFDM symbols or SC-FDMA symbols in the time domainby N^(RB) _(sc) consecutive subcarriers in the frequency domain.Therefore, one PRB includes N^(DL/UL) _(symb)×N^(RB) _(sc) REs.

Each RE in the resource grid may be uniquely identified by an index pair(k, 1) in a slot. k is a frequency-domain index ranging from 0 toN^(DL/UL) _(RB)×N^(RB) _(sc)−1 and 1 is a time-domain index ranging from0 to N^(DL/UL) _(symb)−1.

FIG. 7 illustrates an exemplary structure of a downlink subframe in awireless communication system.

Referring to FIG. 7, each subframe may be divided into a control regionand a data region. The control region starts from the first OFDM symboland includes at least one or more OFDM symbols. The number of OFDMsymbols that are being used as the control region within the subframe istransmitted through the PCFICH (Physical Control Format IndicatorCHannel). The base station may transmit diverse types of controlinformation to the user equipment(s) through the control region. Inorder to transmit the control information, a PDCCH (Physical DownlinkControl CHannel), PCFICH, PHICH (Physical Hybrid automatic retransmitrequest Indicator CHannel), and so on may be assigned to the controlregion.

The base station may transmit data designated for the user equipment oruser equipment group through the data region. Herein, the data beingtransmitted through the data region may also be referred to as userdata. In order to transmit the user data, a PDSCH (Physical DownlinkShared CHannel) may be allocated to the data region. The user equipmentmay decode the control information being transmitted through the PDCCH,so as to be capable of reading the data being transmitted through thePDSCH. For example, information indicating which user equipment or userequipment group the data of the PDSCH is being transmitted to,information indicating how the user equipment or user equipment group isto receive and decode the PDSCH data, and so on may be included in thePDCCH and then transmitted.

The PDCCH carries a transport format of a DL-SCH (Downlink SharedChannel) and a resource allocation (or assignment) information, resourceallocation information of a UL-SCH (Uplink Shared Channel), paginginformation on a PCH (paging channel), system information on the DL-SCH,allocation information of a higher layer control message, such as arandom access response, which is transmitted on the PDSCH, a group of Txpower control commands for each of the UEs within a random UE group,information on a VoIP (voice over IP) activation, and so on. MultiplePDCCHs may be transmitted in the control region. The UE may monitor themultiple PDCCHs and may detect its own PDCCH. In the PDCCH, the size andpurpose of the control information may vary depending upon a DCI(downlink control indicator) format, and the size may also varydepending upon the coding rate.

The DCI format may be independently applied for each UE, and the PDCCHof multiple UEs may be multiplexed in one subframe. The PDCCH of each UEmay be independently channel-coded so that a CRC (cyclic redundancycheck) can be added to the respective PDCCH. The CRC is masked with aunique identifier of each UE so that each UE can receive its respectivePDDCH. However, since the UE is essentially unaware of the position towhich its PDCCH is being transmitted, the UE is required to performblind detection (also referred to as blind decoding) on all of thePDCCHs of the corresponding DCI format, until the UE receives the PDCCHhaving the identifier of the corresponding UE.

There are a variety of DCI formats, for example, format 0 for schedulinga physical uplink shared channel (PUSCH), format 1 for scheduling onePDSCH codeword, format 1A for compact scheduling of one PDSCH codeword,format 1B for compact scheduling of Rank-1 transmission of a singlecodeword in a spatial multiplexing mode, format 1C for very compactscheduling of a downlink shared channel (DL-SCH), format 1D forscheduling a PDSCH in a multi-user spatial multiplexing mode, format 2for scheduling a PDSCH in a closed-loop spatial multiplexing mode,format 2A for scheduling a PDSCH in an open-loop spatial multiplexingmode, format 3 for transmitting a transmission power control (TPC)command of 2-bits power control for PUCCH and PUSCH, and format 3A fortransmitting a TPC command of 1-bit power control for PUCCH and PUSCH.

Hereinafter, in the description of the present invention, thetransmission of data for a UE may be expressed as a PDSCH transmission,and the transmission of control information related to the data may beexpressed as a PDCCH transmission, for simplicity.

FIG. 8 illustrates an exemplary uplink subframe in a wirelesscommunication system.

Referring to FIG. 8, an uplink subframe may be divided into a controlregion and a data region in the frequency domain. In order to carry UCI(uplink control information), one or more PUCCHs (physical uplinkchannels) may be allocated to the control region. In order to carry userdata, one or more PUSCHs (physical uplink shared channels) may beallocated to the data region. When the UE adopts the SC-FDMA scheme forthe uplink transmission, in order to maintain the single carriercharacteristic, the PUCCH and the PUSCH cannot be transmittedsimultaneously. The PUCCH for one UE may be allocated to an RB pairwithin the subframe, and the RBs belonging to the RB pair may eachoccupy a different subcarrier within two slots. Herein, it may beexpressed that an RB pair allocated to the PUCCH performsfrequency-hopping at a slot boundary of the PUCCH being allocated asdescribed above.

Meanwhile, a reference signal (RS) refers to a predefined signal with aspecial waveform known to both the BS and the UE, transmitted from theBS to the UE or from the UE to the BS.

RSs are largely classified into dedicated reference signals (DRSs) andcommon reference signals (CRSs). A CRS is transmitted in every DLsubframe in a cell supporting PDSCH transmission. CRSs are used for bothpurposes of demodulation and measurement and shared among all UEs withinthe cell. A CRS sequence is transmitted through every antenna portirrespective of layers. A DRS is usually used for demodulation,dedicated to a specific UE. The CRSs and DRSs are also calledcell-specific RSs and DMRSs, respectively. The DMRSs are also calledUE-specific RSs.

FIG. 9 is a conceptual view of DRS transmission. Particularly, atransmitter for transmitting precoded RSs is illustrated in FIG. 9, byway of example.

A DRS is dedicated to a particular UE and thus other UEs are not allowedto use the DRS. DRSs used for data demodulation at a specific UE may beclassified into precoded RSs and non-precoded RSs. For example, The UEmay demodulate a received data signal by arranging the data signal atpredetermined positions on a signal constellation according to apredetermined modulation scheme, based on DRSs received along with thedata signal.

FIG. 10 illustrates exemplary DRS patterns in an LTE system.Specifically, FIG. 10( a) illustrates a DRS pattern for a subframe witha normal CP and FIG. 10( b) illustrates a DRS pattern for a subframewith an extended CP. In FIG. 10, ‘1’ represents the position of an OFDMsymbol in a slot.

REs on which DRSs can be transmitted (i.e. DRS REs) are generallypreset, among the REs of an RB or an RB pair. Thus, a UE has only todetect a DRS(s) from an RE(s) at a preset position(s) among the REs ofeach RB or RB pair. For example, referring to FIG. 10, a BS transmitsDRSs in one or more RB pairs through antenna port 5 in the pattern ofFIG. 10( a) or 10(b). Hereinbelow, the positions of DRS REs in an RB orRB pair will be referred to as a DRS pattern in describing embodimentsof the present invention.

In the LTE system supporting up to a maximum of one layer, a BSsimultaneously transmits DRSs for demodulation of the layer and CRSs forestimation of a channel between a UE and the BS. If DRS is transmittedalong with CRS, RS overhead is relatively increased as compared to thecase of only CRS transmission. Specifically, it is possible to transmitmultiple layers in the LTE-A system, such that CRS overhead is increasedand data transmission efficiency is deteriorated. To avert this problem,the LTE-A system that can transmit more layers than the LTE system usesDRS and CSI-RS (channel state information RS) instead of CRSs thatincrease transmission overhead according to the number of physicalantenna ports. The CSI-RSs are RSs introduced due to the fact thatchannel state does not greatly vary over time. Unlike a CRS transmittedin every subframe, the CSI-RS is transmitted at a transmission intervalof a plurality of subframes. Due to such a transmission property of theCSI-RS, CSI-RS transmission overhead is lower than CRS transmissionoverhead. CSI-RS is a reference signal transmitted for channelestimation, and thus a demodulation RS for enabling the UE to demodulatereception data must be transmitted to the UE. For this purpose, DRS isutilized in the LTE-A system. For convenience of description, DRS willhereinafter be referred to as DMRS in the following embodiments.

FIG. 11 is a diagram illustrating DMRS patterns of antenna ports #7˜#10.

In the case of multi-layer transmission, DMRS must be transmitted perlayer, such that the number of DMRSs is increased in proportion to thenumber of layers. Assuming that DMRSs are transmitted through differentresource elements (REs) within one RB pair, the number of RS REs isincreased in proportion to the number of layers, such that datatransmission efficiency is deteriorated. Therefore, if multiple DMRSsmust be transmitted to reduce RS transmission overhead, one or moreDMRSs are generally multiplexed and transmitted through a predeterminednumber of REs.

A base station (BS) capable of supporting a maximum of 4 transmissionlayers may multiplex and transmit a maximum of 4 transmission layers onone data RE. If the BS multiplexes and transmits 4 layers, it transmitsnot only the four layers but also four DMRSs corresponding to the fourlayers, respectively, and used for demodulation of each layer. The fourDMRSs can be respectively transmitted in REs of two groups through 4antenna ports. Referring to FIG. 11, antenna ports #7˜#10 may transmitthe corresponding DMRS through 12 REs within the RB pair. Referring toFIGS. 11( a) and 11(b), a radio resource for DMRS transmission of theantenna port #7 is identical to a radio resource for DMRS transmissionof the antenna port #8. That is, DMRS of the antenna port #7 and DMRS ofthe antenna port #8 are identical to a radio resource for DMRStransmission of the antenna port #9 and a radio resource for DMRStransmission of the antenna port #10, as shown in FIGS. 11( c) and11(d). However, a radio resource (hereinafter referred to as ‘DMRSresource group 1’) for DMRS transmission of the antenna ports #7 and #8is different from a radio resource (hereinafter referred to as ‘DMRSresource group 2’) for DMRS transmission of the antenna ports #9 and#10. That is, DMRS of the antenna port #7 and DMRS of the antenna port#8 are multiplexed to DMRS resource group 1, such that they can besimultaneously transmitted. DMRS of the antenna port #9 and DMRS of theantenna port #10 are multiplexed to DMRS resource group #2, such thatthey can be simultaneously transmitted.

If multiple DMRSs are multiplexed in a predetermined radio resource asshown in FIG. 11, an orthogonal cover code (OCC) may be utilized todiscriminate between DMRSs. For example, if DMRS is spread using the OCChaving the length of 2, a maximum of two different DMRSs can betransmitted through one RE. In another example, if DMRS is spread usingan OCC having the length of 4, a maximum of 4 different DMRSs can bemultiplexed through one RE. For example, the Walsh-Hadamard code may beused as a representative OCC.

From among REs of one RB or REs of one RB pair, an aggregate of REs fortransmission of DMRSs that can be spread by OCCs so as to be identifiedfrom each other is referred to as a CDM group. Referring to FIG. 11,DMRS resource group 1 corresponding to an aggregate of REs to whichDMRSs of antenna ports #7 and #8 are allocated may configure one CDMgroup, and DMRS resource group 2 corresponding to an aggregate of REs towhich DMRSs of the antenna ports #9 and #10 are allocated may configureanother CDM group. In one pair of contiguous REs (hereinafter referredto as an RB pair), each CDM group of FIG. 11 includes 12 REs.

FIG. 12 shows resource elements (REs) obtained when DMRSs aremultiplexed using OCCs having the length of 2.

In the system capable of supporting a MIMO having a maximum rank of 4, amaximum of four DMRS sequences may be transmitted through two codedivision multiplexing (CDM) groups. Two DMRSs for each DRM group may bemultiplexed by 2 OCC sequences each having the length of 2.

Antenna ports for transmitting DMRSx, DMRSy, DMRSz and DMRSw are definedas DMRS port X, DMRS port Y, DMRS port Z, and DMRS port W, respectively.In addition, it is assumed that two OCC sequences each having a lengthof 2 are represented by [1 1] and [1 −1]. The two OCC sequences maycorrespond to row-directional sequences of a 2×2 matrix shown in FIG.12.

Referring to FIG. 12, DMRSx is spread by a sequence [1 1], DMRSy isspread by a sequence [1 −1], and the spread results may be allocated toCDM Group 1. DMRSz is spread by either one of the sequences [1 1] and [1−1], DMRSw is spread by the remaining sequences, and the spread resultsmay be allocated to CDM Group 2.

The RB pair shown in FIG. 12 may include a total of 4 DMRS symbols#1˜#4. Some parts of the DMRSx spread by the sequence [1 1] and someparts of the DMRSy spread by the sequence [1 −1] may be allocated toDMRS Symbol 1. For example, DMRSx is spread to [1 1]×DMRSx=[DMRSx DMRSx]by the sequence [1 1], and DMRSy is spread to [1 −1]×DMRSy=[DMRSy−DMRSy] by the sequence [1 −1], such that first elements (DMRSx andDMRSy) may be allocated to DRMS Symbol 1 and second elements (DMRSx and−DMRSy) may be allocated to DMRS Symbol 2. In other words,(1×DMRSx)+(1×DMRSy) may be allocated to DMRS Symbol 1, and(1×DMRSx)+(−1×DMRSy) may be allocated to DMRS Symbol 2.

FIG. 13 shows resource elements (REs) obtained when DMRSs aremultiplexed using OCCs having a length of 4.

In a MIMO system for supporting a maximum rank of 8, a maximum of 8 DMRSsequences may be transmitted through 2 CDM groups. Four DMRSs may bemultiplexed per CDM group, and may also be multiplexed by four OCCsequences having a length of 4. It is assumed that antenna ports fortransmitting DMRSx, DMRSy, DMRSz, and DMRSw are represented by DMRS portX, DMRS port Y, DMRS port Z, and DMRS port W, respectively. In addition,four OCC sequences having a length of 4 are represented by [1 1 1 1], [1−1 1 −1], [1 1 −1 −1], and [1 −1 −1 1], respectively. The four OCCsequences may correspond to row-directional sequences of a 4×4 matrixshown in FIG. 12.

Referring to FIG. 13, DMRSx is spread by a sequence [1 1 1 1], DMRSy isspread by a sequence [1 −1 1 −1], DMRSz is spread by a sequence [1 1 −1−1], DMRSw is spread by a sequence [1 −1 −1 1], and the spread resultsmay be allocated to CDM Group 1. Four DMRSs different from DMRSx, DMRSy,DMRSz, and DMRSw may be spread by [1 1 1 1], [1 −1 1 −1], [1 1 −1 −1],and [1 −1 −1 1], such that the spread results may be allocated to CDMGroup 2.

The RB pair shown in FIG. 13 includes a total of 4 DMRS symbols #1˜#4.Some parts of DMRSx, DMRSy, DMRSz, and DMRSw spread by the sequences [11 1 1], [1 −1 1 −1], [1 1 −1 −1], and [1 −1 −1 1] may be allocated toDMRS Symbol 1. For example, DMRSx is spread to [1 1 1 1]×DMRSx=[DMRSxDMRSx DMRSx DMRSx] by the sequence [1 1 1 1], DMRSy is spread to [1 −1 1−1]×DMRSy=[DMRSy−DMRSy DMRSy−DMRSy] by the sequence [1 −1 1 −1], DMRSzis spread to [1 1 −1 −1]×DMRSz=[DMRSz DMRSz−DMRSz−DMRSz] by the sequence[1 1 −1 −1], and DMRSw is spread to [1 −1 −1 1]×DMRSw=[DMRSw−DMRSw−DMRSwDMRSw] by the sequence [1 −1 −1 1]. For example, from among the spreadDMRS sequences, first elements (DMRSx, DMRSy, DMRSz, DMRSw) may beallocated to DMRS Symbol 1, second elements (DMRSx, −DMRSy, DMRSz,−DMRSw) may be allocated to DMRS Symbol 2, third elements (DMRSx, DMRSy,−DMRSz, −DMRSw) may be allocated to DMRS Symbol 3, and fourth elements(DMRSx, −DMRSy, −DMRSz, DMRSw) may be allocated to DMRS Symbol 4. Thatis, a component denoted by (1×DMRSx)+(1×DMRSy)+(1×DMRSz)+(1×DMRSw) isallocated to DMRS Symbol 1, a component denoted by(1×DMRSx)+(−1×DMRSy)+(1×DMRSz)+(−1×DMRSw) is allocated to DMRS Symbol 2,a component denoted by (1×DMRSx)+(1×DMRSy)+(−1×DMRSz)+(−1×DMRSw) isallocated to DMRS Symbol 3, and a component denoted by(1×DMRSx)+(−1×DMRSy)+(−1×DMRSz)+(1×DMRSw) is allocated to DMRS Symbol 4.

The DMRS pattern illustrated in FIG. 12 and the DMRS pattern illustratedin FIG. 13 may be carried out simultaneously or only either of them maybe carried out in a wireless communication system. For example, the DMRSpattern of FIG. 12 may be used for a BS to multiplex one to four layers,for transmission, whereas the DMRS pattern of FIG. 13 may be used for aBS to multiplex five to eight layers, for transmission. In anotherexample, one to eight layers may be multiplexed and transmitted usingthe DMRS pattern of FIG. 13. Notably, since a length of OCCs varies withthe total number of layers transmitted by a BS in the former case,information indicating the total number of layers transmitted by the BSor a length of OCCs used for multiplexing the layers should be signaledto a UE explicitly or implicitly so that the UE may detect its layerusing an OCC. For convenience of description and better understanding ofthe present invention, the embodiments of the present invention willhereinafter be described using an exemplary case in which the BSperforms CDM multiplexing of fifth to eighth layers using OCCs having alength of 4 and transmits the CDM-multiplexed result. However, thefollowing embodiments of the present invention can also be applied to anexemplary case in which one to four layers are multiplexed using OCCshaving a length of 2.

In order for a UE to demodulate data transmitted to the UE, the UE canhave to separate each transmission layer used for transmission of thedata to the UE using channel information obtained through DMRS(s)transmitted by a BS along with the data. For this purpose, the UE has toseparate a DMRS for its own data. In order to enable the UE todiscriminate between a port that has transmitted a layer transmitted bythe BS and a DMRS corresponding to the layer, the number of layerstransmitted to a UE and a DMRS transmitted for the layers willhereinafter be described with reference to the following embodiments.

<Mapping of Layer to DMRS>

If a CDM group and an OCC used for DMRS extension for a UE from among 4OCCs contained in the CDM group are specified, the UE may recover theDMRS. That is, provided that the UE can recognize whether its own DMRShas been transmitted from REs of CDM Group 1 or REs of CDM Group 2, andcan also recognize whether its own DMRS has been spread with a certainOCC and the spread result has been multiplexed to the corresponding CDMgroup, the UE can detect its own DMRS that has been transmitted from theBS. For example, if the UE can recognize that its own DMRS has beenspread with a first OCC within the CDM Group 1, the UE may multiply thefirst OCC by a signal transmitted on REs of the CDM Group 1, such thatit can detect its own DMRS. For convenience of description and betterunderstanding of the present invention, DMRSs spread by {1st OCC, 2ndOCC, 3rd OCC, 4th OCC} and transmitted in CDM Group 1 are represented by{1C, 2C, 3C, 4C}, respectively, and DMRSs spread by {1st OCC, 2nd OCC,3rd OCC, 4th OCC} and transmitted in CDM Group 2 are represented by {1D,2D, 3D, 4D}, respectively.

1. Case in which the Mapping Relationship Between all Layers and allDMRSS is Predetermined:

The mapping relationship between all DMRSs and all layers ispredetermined, and DMRS may be specified on the basis of thepredetermined mapping relationship. The mapping relationship between 8DMRSs {1C, 2C, 3C, 4C, 1D, 2D, 3D, 4D} and 8 layers is represented bythe following Table 1.

TABLE 1 Layer allocation rules for codes in two CDM groups Lay- Lay-Lay- Lay- Lay- Lay- Lay- Lay- er-0 er-1 er-2 er-3 er-4 er-5 er-6 er-7Type 1 1C 2C 1D 2D 3C 4C 3D 4D Type 2 1C 2C 1D 2D 3C 3D 4C 4D Type-3 1C2C 1D 2D 3D 3C 4D 4C Type-4 1C 2C 1D 2D 4C 4D 3C 3D Type-5 1C 2C 3C 4C1D 2D 3D 4D

If K_(max) transmission antennas are contained in the BS, K_(max) layersfrom Layer-1 to Layer-K_(max) may be defined, and K_(max) DMRSs may bemapped to K_(max) layers on a one to one basis. For example, referringto Type-1, DMRSs are mapped to 8 layers from Layer-0 to Layer-7 in theorder of 1C→2C→1D→2D→3C→4C→3D→4D. That is, DMRSs of Layer-0, Layer-1,Layer-4, and Layer-5 are spread to a first OCC, a second OCC, a thirdOCC and a fourth OCC, respectively, such that the spread DMRSs may betransmitted within CDM Group 1 and DMRSs of Layer-2, Layer-3, Layer-6and Layer-7 are spread to 1^(st) OCC, 2^(nd) OCC, 3^(rd) OCC, and 4^(th)OCC, respectively, such that the spread results are transmitted in CDMGroup 2. In so far as layers are mapped to DMRSs on a one to one basis,other mapping relationships different from Type-1 to Type-5 mappingrelationships of Table 1 may be defined.

Meanwhile, layers from Layer-0 to Layer-7 shown in Table 1 are logicalindexes assigned to discriminate among 8 layers for convenience ofdescription. Since one layer is transmitted per antenna port, themapping relationship between layers and DMRSs may also be considered tobe the mapping relationship between antenna ports and DMRSs. Providedthat the antenna port for transmitting the DMRS is referred to as a DMRSport, Table 1 may also be represented by the following Table 2.

TABLE 2 DMRS port (Port) allocation rules for codes in two CDM groupsPort- Port- Port- Port- Port- Port- Port- Port- 0 1 2 3 4 5 6 7 Type-11C 2C 1D 2D 3C 4C 3D 4D Type-2 1C 2C 1D 2D 3C 3D 4C 4D Type 3 1C 2C 1D2D 3D 3C 4D 4C Type-4 1C 2C 1D 2D 4C 4D 3C 3D Type-5 1C 2C 3C 4C 1D 2D3D 4D

If K_(max) transmission antenna ports are contained in the BS, K_(max)antenna ports may respectively transmit DMRSs, and K_(max) DMRSs may bemapped to K_(max) antenna ports on a one to one basis. For example, ascan be seen from Type-2, Port-0, Port-1, Port-4, and Port-6 may indicatethat a DMRS spread by the 1^(st) OCC, a DMRS spread by the 2^(nd) OCC, aDMRS spread by the 3^(rd) OCC, and a DMRS spread by the 4^(th) OCC aretransmitted in CDM Group 1, and Port-2, Port-3, Port-5, and Port-7 mayindicate that a DMRS spread by the 1^(st) OCC, a DMRS spread by the2^(nd) OCC, a DMRS spread by the 3^(rd) OCC, and a DMRS spread by the4^(th) OCC are transmitted in CDM Group 2.

In a wireless communication system, a single mapping type (for example,Type-2) may be defined, and all UEs and all BSs for use in the wirelesscommunication system may transmit/receive DMRSs using the single mappingtype. Differently from the above-mentioned description, a plurality ofmapping types may be defined in the wireless communication system, andthe mapping types may be UE-specifically or BS-specifically used. If themapping types may be UE-specifically or BS-specifically used, themapping types available to a specific UE or a specific BS may beimplicitly determined. Alternatively, information indicating whetherDMRS is transmitted on the basis of a certain mapping type from among aplurality of mapping types may be signaled from a UE to a BS or from aBS or a UE so as to determine the mapping type. Embodiment 1-1 andEmbodiment 1-2 may be applied to the case in which only one mapping typeis defined in the wireless communication system. In addition, Embodiment1-1 and Embodiment 1-2 may also be applied to another case in whichseveral mapping types are defined and the multiple mapping types may beUE-specifically or BS-specifically used.

Based on a predetermined mapping relationship between layers and DMRSs,one or more layers may be allocated to one UE using Embodiments 1-1 to1-3.

Embodiment 1-1 Fixed DMRS set is Used Per Rank

The DMRS set may be predefined according to the number (i.e., rank) oflayers allocated to the UE. For example, in the case of Rank-3transmission, DMRSs corresponding to layers from Layer-0 to Layer-2 maybe utilized in Table 1. If the mapping relationship of Type-1 isachieved between layers and DMRSs, DMRSs of {1C, 2C, 1D} may be adaptedto transmit user data for one or more UEs receiving Rank-3 transmission.In so far as a DMRS set used for a specific rank is predefined, otherDMRS sets (for example, {1D, 2D, 3D}) may be utilized for Rank-3transmission.

Based on signaling from the BS or blind decoding of the UE, the UE canrecognize how many layers have been allocated to the UE. Accordingly,according to Embodiment 1-1 in which a DMRS set is fixed per rank, theUE can detect its own DMRS(s) from among DMRSs transmitted from the BS.Moreover, the UE can demodulate UE data transmitted from one or morelayers on the basis of the detected DMRS.

Embodiment 1-2 Contiguous DMRS(s) Starting from a Specific Layer areUsed

An arbitrary DMRS set may be utilized to transmit the same rank duringeach transmission time. In this case, information indicating which DMRSset is used is transferred to the UE. For example, not only a rank forthe UE but also a start DMRS may be adapted to indicate a DMRS set usedfor data transmission to the UE. Referring to Type-1 of Table 1, the UEmay demodulate data transmitted to the UE using 4 DMRSs (i.e., {1D, 2D,3C, 4D}) starting from 1D, namely, the UE may demodulate UE data through4 layers corresponding to 4 DMRSs. In another example, provided that astart DMRS is set to a DMRS corresponding to Layer-6 of Table 1 and arank is set to 3 (Rank-3), 3 DMRSs may start from 3D in Type-1 and maybe cyclically allocated to the UE. That is, the UE may use the allocatedDMRSs {3D, 4D, 1C} to demodulate a reception layer.

1-3. Mapping of Layer to DMRS According to Feedback Mode

The mapping relationship between layers and DMRSs may be separatelydefined according to the feedback mode. For example, during the SU-MIMOfeedback mode, fixed DMRS(s) may be allocated to layer(s) for a UE asshown in Embodiment 1-1. During the MU-MIMO feedback mode, DMRS(s) notfixed as shown in Embodiment 1-2 are allocated to layer(s) for a UE, andinformation specifying the allocated DMRS(s) may be signaled to the UE.

2. Case in which the mapping relationship between layers and DMRSs isnot predefined:

Although the mapping relationship between a layer or DMRS port and aDMRS is not defined, DMRS(s) for a UE may be indicated.

2-1: Indicating Rank and CDM Group

It is assumed that one UE receives a maximum of 4 layers and only oneCDM group can be adapted to transmit data toward one UE. In addition,assuming that one CDM group cannot be allocated to different UEs, the BScan transmit a plurality of layers to a maximum of 2 UEs using a maximumof 2 CDM groups. In this case, the BS may inform the UE of the number oflayers allocated to the UE (i.e., CDM group to which a rank or acorresponding DMRS is transmitted), and the UE may specify DMRS(s) fordata demodulation. For example, if the BS transmits three layers to aUE1 using CDM Group 1, the BS may inform the UE1 of informationindicating Rank-3 and information indicating CDM Group 1. Based on theabove-mentioned information, the UE1 can recognize that 3 DMRSs for theUE1 have been transmitted through CDM Group 1. The UE1 may detect threeDMRSs from the signal received from CDM Group 1 using three OCCs {1stOCC, 2nd OCC, 3rd OCC} from among four OCCs {1st OCC, 2nd OCC, 3rd OCC,4th OCC}. On the other hand, if the UE transmits data to a UE1 and atthe same time transmits two layers to a UE2 using CDM Group 2, the BSmay inform the UE2 of information indicating Rank-2 and informationindicating CDM Group 2. The UE2 may detect two DMRSs from the signalreceived from the CDM Group 2 using two OCCs {1st OCC, 2nd OCC} fromamong four OCCs {1st OCC, 2nd OCC, 3rd OCC, 4th OCC} available to CDMGroup 2.

On the other hand, information indicating a CDM group may be composed ofRRC signaling, and may be signaled to a UE at a longer period. Rankindication information may be signaled to a UE at each downlinktransmission.

2-2: Indication of Rank and Start DMRS, and CDM Group

The BS may inform the UE of a rank allocated to the UE, a CDM group forDMRS transmission, and a start DMRS, such that the UE can specifyDMRS(s) for data demodulation. The start DMRS of one CDM group may besignaled by specifying the start OCC from among OCCs available to oneCDM group.

Rank and start DMRS, and DMRSs transmitted in the used CDM group may bechanged according to a method for allocating the CDM group to the UE.Therefore, Embodiment 2-2 illustrates one case (1) in which only one CDMgroup can be allocated to one UE and another case (2) in which one ortwo CDM groups can be allocated to one UE.

(1) Case in which One CDM Group is Allocated to One UE

Only one CDM group can be allocated per UE. That is, the BS may beconfigured not to allocate a CDM group allocated to one UE to anotherUE. That is, it is assumed that only one CDM group is used for datatransmission toward one UE and the CDM group for use in the UE cannot beutilized by another UE. In this case, the BS can transmit a plurality oflayers to a maximum of two UEs using a maximum of two CDM groups. The BSmay inform the UE of the number of layers allocated to the UE (i.e., arank and a used CDM group) and a start DMRS from among DMRSs used in theCDM group, such that the UE can specify DMRS(s) needed for datademodulation. For example, if the BS multiplexes 3 layers using threeDMRSs {1C, 2C, 3C} and transmits the three multiplexed DMRSs to a UE1,the BS may inform the UE1 of information indicating Rank-3, informationindicating CDM Group 1, and information indicating a start DMRS (1C).Based on the above-mentioned information, the UE1 may detect DMRSscapable of being transmitted in CDM Group 1. For example, the UE maydetect three DMRSs {1C, 2C, 3C} starting from DMRS 1C from among fourDMRSs {1C, 2C, 3C, 4C}. The UE1 may demodulate data transmitted for theUE1 using the detected DMRS. On the other hand, in the case where dataof the UE1 is transmitted and at the same time the BS multiplexes twolayers using {2D, 3D} and transmits the two multiplexed layers to theUE2, the BS may inform the UE2 of information indicating Rank-2,information indicating CDM Group 2, and information indicating a startDMRS (2D). Based on the above-mentioned information, the UE2 may detectDMRSs (for example, two DMRSs {2D, 3D} starting from 1D from among fourDMRSs {1D, 2D, 3D, 4D}) capable of being transmitted in the CDM group 2.The UE2 can demodulate data transmitted for the UE2 using the detectedDMRS.

Meanwhile, it is assumed that only one CDM group is used for datatransmission toward one UE and a CDM group used for the UE can also beapplied to another UE. In this case, the BS may multiplex data for amaximum of 8 UEs to a predetermined radio resource using a maximum of 2CDM groups. In this case, the BS may inform the UE of the number oflayers allocated to the UE (i.e., a rank and a used CDM group) and astart DMRS from among DMRSs used in the CDM group, such that the UE canspecify DMRS(s) needed for data demodulation. For example, it is assumedthat the BS transmits three layers allocated to a UE1 and three DMRSs{1C, 2C, 3C}, transmits DMRSs {2D, 3D} together with two layersallocated to a UE2, and transmits a DMRS 4D along with one layerallocated to a UE3. The BS may inform the UE1 of information indicatingRank-3, information indicating CDM Group 1, and information indicating astart DMRS (1C), may inform the UE2 of information indicating Rank-2,information indicating CDM Group 2, and information indicating a startDMRS (2D), and may inform a UE3 of information indicating a start DMRS(1C), may inform the UE2 of information indicating Rank-1, informationindicating CDM Group 2, and information indicating a start DMRS (4D).From among DMRSs capable of being transmitted on CDM Group 1 on thebasis of information transmitted to the UE1, the UE1 detects three DMRSs{1C, 2C, 3C} starting from 1C, and can demodulate data transmitted forthe UE1. In addition, the UE2 detects two DMRSs {2D, 3D} from amongDMRSs {1D, 2D, 3D, 4D} capable of being transmitted on CDM Group 2 onthe basis of information transmitted to the UE2, such that it candemodulate data transmitted for the UE2. In addition, the UE3 can detecta DMRS 4D from among DMRSs capable of being transmitted on CDM Group 2on the basis of information transmitted to the UE3, such that it candemodulate data transmitted for the UE3.

(2) One or Two CDM Groups are Allocated to CDM Group of One UE

If necessary, the CDM group allocated to another UE may also beallocated to one UE. That is, the BS may allocate the CDM groupallocated to one UE to another UE as necessary.

It is assumed that one or two CDM groups can be used to transmit data toone UE. The BS can multiplex data for a maximum of 8 UEs using a maximumof two CDM groups. For example, it is assumed that the BS allocatesthree layers to a UE3, and not only three layers but also DMRSs {3C, 4C,1D} corresponding to the three layers can be transmitted to the UE3. TheBS may inform the UE3 of information indicating CDM Group 1, informationindicating the number (i.e., 2) of DMRSs used in the CDM Group 1,information indicating a start DMS (1C) of the CDM Group 1, informationindicating CDM Group 2, information indicating the number (i.e., 1) ofDMRSs used in the CDM Group 2, and information indicating a start DMRS(1D) of the CDM Group 2. The UE3 detects two DMRSs {3C, 4C} startingfrom 3C from among DMRSs {1C, 2C, 3C, 4C} capable of being transmittedon CDM Group 1 on the basis of the above-mentioned information, anddetects one DMRS (1D) defined as a start DMRS from among DMRSs {1D, 2D,3D, 4D} capable of being transmitted on CDM Group 2, such that it candemodulate data transmitted for UE3.

In accordance with Embodiment 2-2 of the present invention, informationindicating the CDM group is configured by RRC signaling such that it canbe signaled to a UE in a long-term manner. Rank information indicatingthe rank and start DMRS information indicating the start DMRS (or startOCC) may be signaled to the UE at every downlink transmission.

2-3: DMRS Set Indication Rule According to UE Class

A maximum number of supportable ranks may be changed per UE. It ispossible to classify classes for UEs on the basis of a supportable rank,and a method for informing the UE of the corresponding DMRS according tothe UE class may be changed. In association with a UE capable ofreceiving a maximum of 4 layers, a CDM group and a rank may be signaledfrom the BS according to the embodiment 2-1. In association with a UEcapable of receiving at least 5 layers, according to embodiment 2-2,information indicating a CDM group and a rank may be signaled from theBS, and information indicating a start DMRS from among DMRSs used in theCDM group may also be signaled from the BS.

3. Use of DMRS Subset

If K_(max) DMRSs are defined for K_(max) layers, the subset of theK_(max) DMRSs may be UE-specifically or cell-specifically used asnecessary. Information indicating which subset from among DMRSs is usedmay be indicated to a bitmap or a predefined group index. Informationindicating a DMRS subset may be configured by RRC or higher layersignaling, and the resultant information may be transferred to the UE.

For example, provided that Type-1 of Table 1 or Table 2 is used in apredetermined system, DMRSs {1C, 2C, 1D, 2D, 3C, 4C, 3D, 4D} may beutilized for 8 layers or 8 DMRS ports. The BS may limit available DMRSsfrom among all DMRSs {1C, 2C, 1D, 2D, 3C, 4C, 3D, 4D} to a subset ofDMRSs {1D, 2D, 3D, 4D}, and may inform the UE of information indicatingthe DMRS subset. The UE may feed back transmission of a maximum ofRank-4, and the BS may transmit DMRS(s) corresponding to one or morelayers transmitted to the UE on the basis of the layer-to-DMRS mappingrelationship {1D: Layer-0, 2D: Layer-1, 3D: Layer-2, 4D: Layer-3}. Inaddition, the BS may inform the UE of information (hereinafter referredto as DMRS indication information) indicating a DMRS that has beentransmitted for the UE according to Embodiment 1-1 or Embodiment 1-2.The UE may detect a DMRS corresponding to a layer allocated to the UEaccording to Embodiment 1-1 or Embodiment 1-2, and the UE may demodulatea layer allocated to the UE using the detected DMRS. The BS may transmitor receive DMRSs to the UE on the basis of the mapping relationship {1D:Port-0, 2D: Port-1, 3D: Port-2, 4D: Port-3} between DMRS ports andDMRSs.

In another example, available DMRSs from among DMRSs {1C, 2C, 3C, 4C} ofCDM Group 1 and DMRSs {1D, 2D, 3D, 4D} of CDM Group 2 are limited to asubset of DMRSs {1C, 2C, 1D, 2D}, and information regarding the DMRSsubset may be signaled to the UE. The BS may allocate a maximum of 4layers to the UE using the DMRS subset {1C, 2C, 1D, 2D}. If the BStransmits one or more layers and DMRSs for demodulating the allocatedlayers to the UE, the BS may inform the UE of indication informationregarding DMRSs allocated for the UE according to any one of embodiments2-1 to 2-3. In accordance with any one of Embodiments 2-1 to 2-3, the UEmay detect a DMRS corresponding to a layer allocated to the UE on thebasis of the DMRS indication information, and the UE may demodulate theallocated layer on the basis of the detected DMRS.

In accordance with any one of the above-mentioned embodiments 1-1 to 3,the BS processor 400 b may allocate a DMRS for each layer to betransmitted to a UE. In addition, according to any one of embodiments1-1 to 3, the BS processor 400 b may spread a DMRS allocated to the UEwith a predetermined OCC, and multiplex the spread result in apredetermined CDM group. In order to detect DMRS(s) allocated to the UE,the BS processor 400 b may control the BS transmitter 100 b to generateinformation for specifying/indicating DMRS(s) according to any one ofembodiments 1-1 to 3, as well as to transmit the generated informationto the UE. For example, the BS processor 400 b may generate informationindicating a mapping type, information indicating OCC(s) used forspreading the DMRS(s), and information indicting CDM group(s) to whichthe DMRS(s) are transmitted. Under control of the BS processor 400 b,the BS transmitter 100 b transmits each DMRS port allocated to the UE toeach layer and each DMRS of the UE. In addition, under the control ofthe BS processor 400 b, the BS transmitter 100 b may transmitinformation for specifying/indicating the DMRS to the UE.

Under the control of the UE processor 400 a, the UE receiver 300 a candetect DMRSs transmitted for the UE according to any one of embodiments1-1 to 3. The UE processor 400 a may control the UE receiver 300 a todetect DMRS(s) of the UE according to any one of embodiments 1-1 to 3.In accordance with any one of embodiments 1-1 to 3, if the BS transmitsinformation for specifying/indicating DMRS(s), the UE receiver 300 areceives the above-mentioned information and transmits the receivedinformation to the UE processor 400 a. In accordance with thecorresponding embodiment, the UE processor 400 a may control the UEreceiver 300 a to detect the DMRS on the basis of the above-mentionedinformation. The UE processor 400 a may control the UE receiver 300 a todetect/receive the corresponding layer on the basis of the detectedDMRS.

<DMRS Port Allocation>

The above-mentioned embodiments have disclosed a method for allocating aDMRS to a UE and a method for signaling indication information of theallocated DMRS. In accordance with any one of embodiments 1-1 to 3, theBS allocates DMRS(s) to transmission layer(s), and transmits informationspecifying the allocated DMRS to a UE.

FIG. 14 is a flowchart illustrating a method for processing data using aDMRS.

Referring to FIG. 14, according to any one of embodiments 1-1 to 3, theUE can detect DMRS(s) transmitted thereto (Step S1410). The UE maydemodulate layer(s) allocated/transmitted to the UE on the basis of thedetected DMRS(s) (Step S1420). The UE may recover the demodulatedlayer(s) to one or more codewords (Step S1430).

Table 3 shows the mapping relationship between a codeword and a layerfor spatial multiplexing according to rank.

TABLE 3 Rank Codeword Layer 1 CW-0 layer-0 2 CW-0 layer-0 CW-1 layer-1 3CW-0 layer-0 CW-1 layer-1, layer-2 4 CW-0 layer-0, layer-1 CW-1 layer-2,layer-3 5 CW-0 layer-0, layer-1 CW-1 layer-2, layer-3, layer-4 6 CW-0layer-0, layer-1, layer-2 CW-1 layer-3, layer-4, layer-5 7 CW-0 layer-0,layer-1, layer-2 CW-1 layer-3, layer-4, layer-5, layer-6 8 CW-0 layer-0,layer-1, layer-2, layer-3 CW-1 layer-4, layer-5, layer-6, layer-7

In Table 3, layer indexes are considered to be logical indexes that areassigned in order from a layer belonging to CW-0 to a layer belonging toCW-1 so as to discriminate among layers. Although Table 3 exemplarilyshows transmission of a maximum of 2 codewords, the mapping relationshipbetween a codeword and a layer can also be defined even in the casewhere two or more codewords are transmitted. If the mapping relationshipbetween a codeword and a layer is defined, the embodiments of thepresent invention can be used in the same manner as in transmission of amaximum of two codewords.

If the antenna port for transmitting a specific DMRS is predefined, theUE can detect a DMRS transmitted thereto according to any one of theabove-mentioned embodiments. However, if information as to which port isused to transmit a layer transmission and information as to whichcodeword is mapped to the layer are not defined, the UE cannot recognizea codeword to which the detected layer pertains, such that it isimpossible to properly recover the codeword. For example, as can be seenfrom Table 3, in the case of Rank-4, the BS modulates a codeword CW-0into Layer-0 and Layer-1 and transmits the modulated layers to the UE2.In addition, the BS may modulate a codeword CW-1 into Layer-2 andLayer-3, and transmit the modulated layers to the UE2. In accordancewith any one of embodiments 1-1 to 3, the BS may transmit theabove-mentioned layers together with DMRSs, and may signal indicationinformation of the corresponding DMRS to each UE. The UE1 and the UE2detect a DMRS according to the corresponding embodiment, such that eachof the UE1 and UE2 can detect two layers.

However, if the mapping relationship between a DMRS port and a layer isnot defined, not only information as to which one of antenna ports isused for a layer to be transmitted, but also information as to which oneof codewords is mapped to the layer are unclear, such that it isimpossible to properly recover a codeword only using thelayer-to-codeword mapping relationship. Alternatively, if the mappingrelationship between a DMRS port and a layer is defined, the UE detectsonly a DMRS allocated to the UE, such that it can identify which antennaport has been used to transmit a layer allocated to the UE. In thiscase, the UE can recover a codeword to be transmitted from the BS to theUE on the basis of the layer-to-codeword mapping relationship.

Hereinafter, an antenna port for DMRS transmission is referred to as aDMRS port. In addition, assuming that 8 DMRS ports are mapped to DMRSsas shown in Table 4, the embodiments for the mapping relationshipbetween a DMRS port and a layer will hereinafter be described in detail.For convenience of description and better understanding of the presentinvention, the 8 DMRS ports are indexed from ‘Port-0’ to ‘AntennaPort-7’.

TABLE 4 DMRS port (Port) Port- Port- Port- Port- Port- Port- Port- Port-0 1 2 3 4 5 6 7 DMRS 1C 2C 1D 2D 3C 3D 4C 4D

For example, layer(s) and DMRS port(s) of the codeword are mapped asshown in Table 5 or Table 6.

TABLE 5 Rank Codeword Layer DMRS port (Port) 1 CW-0 layer-0 Port-0 2CW-0 layer-0 Port-0 CW-1 layer-1 Port-1 3 CW-0 layer-0 Port-0 CW-1layer-1, layer-2 Port-1, Port-2 4 CW-0 layer-0, layer-1 Port-0, Port-1CW-1 layer-2, layer-3 Port-2, Port-3 5 CW-0 layer-0, layer-1 Port-0,Port-1 CW-1 layer-2, layer-3, layer-4 Port-2, Port-3, Port-4 6 CW-0layer-0, layer-1, layer-2 Port-0, Port-1, Port-2 CW-1 layer-3, layer-4,layer-5 Port-3, Port-4, Port-5 7 CW-0 layer-0, layer-1, layer-2 Port-0,Port-1, Port-2 CW-1 layer-3, layer-4, layer-5, Port-3, Port-4, Port-5,layer-6 Port-6 8 CW-0 layer-0, layer-1, layer-2, Port-0, Port-1, Port-2,layer-3 Port-3 CW-1 layer-4, layer-5, layer-6, Port-4, Port-5, Port-6,layer-7 Port-7

TABLE 6 Rank Codeword Layer DMRS port (Port) 1 CW-0 layer-0 Port-0 2CW-0 layer-0 Port-0 CW-1 layer-1 Port-1 3 CW-0 layer-0 Port-0 CW-1layer-1, layer-2 Port-2, Port-3 4 CW-0 layer-0, layer-1 Port-0, Port-1CW-1 layer-2, layer-3 Port-2, Port-3 5 CW-0 layer-0, layer-1 Port-0,Port-1 CW-1 layer-2, layer-3, layer-4 Port-2, Port-3, Port-6 6 CW-0layer-0, layer-1, layer-2 Port-0, Port-1, Port-4 CW-1 layer-3, layer-4,layer-5 Port-2, Port-3, Port-6 7 CW-0 layer-0, layer-1, layer-2 Port-0,Port-1, Port-4 CW-1 layer-3, layer-4, layer-5, Port-2, Port-3, Port-6,layer-6 Port-7 8 CW-0 layer-0, layer-1, layer-2, Port-0, Port-1, Port-4,layer-3 Port-5 CW-1 layer-4, layer-5, layer-6, Port-2, Port-3, Port-6,layer-7 Port-7

In Tables 4 to 6, layer indexes are used as logical indexes assigned todiscriminate among layers according to transmission rank. Accordingly,Tables 5 and 6 may also be represented by the mapping relationshipbetween antenna ports and codewords. That is, the embodiment of Table 5may be represented by the mapping relationship between a DMRS port and acodeword as shown in Table 7, and the embodiment of Table 6 may also berepresented by the mapping relationship between a DMRS port and acodeword as shown in Table 8.

TABLE 7 Rank Codeword DMRS port (Port) 1 CW-0 Port-0 2 CW-0 Port-0 CW-1Port-1 3 CW-0 Port-0 CW-1 Port-1, Port-2 4 CW-0 Port-0, Port-1 CW-1Port-2, Port-3 5 CW-0 Port-0, Port-1 CW-1 Port-2, Port-3, Port-4 6 CW-0Port-0, Port-1, Port-2 CW-1 Port-3, Port-4, Port-5 7 CW-0 Port-0,Port-1, Port-2 CW-1 Port-3, Port-4, Port-5, Port-6 8 CW-0 Port-0,Port-1, Port-2, Port-3 CW-1 Port-4, Port-5, Port-6, Port-7

TABLE 8 Rank Codeword DMRS port (Port) 1 CW-0 Port-0 2 CW-0 Port-0 CW-1Port-1 3 CW-0 Port-0 CW-1 Port-2, Port-3 4 CW-0 Port-0, Port-1 CW-1Port-2, Port-3 5 CW-0 Port-0, Port-1 CW-1 Port-2, Port-3, Port-6 6 CW-0Port-0, Port-1, Port-4 CW-1 Port-2, Port-3, Port-6 7 CW-0 Port-0,Port-1, Port-4 CW-1 Port-2, Port-3, Port-6, Port-7 8 CW-0 Port-0,Port-1, Port-4, Port-5 CW-1 Port-2, Port-3, Port-6, Port-7

4. The Mapping of a Codeword to a DMRS Port as Shown in Table 5 or Table7

Referring to Table 5 or Table 7, when transmitting three layers to a UE,the BS transmits one layer corresponding to CW-0 through Port-0, andtransmits two layers corresponding to CW-1 through Port-1 and Port-2.Provided that a transmission DMRS for each port is determined accordingto Table 4, the BS transmits a DMRS 1C at Port-0 and transmits a DMRS 2Cat Port-2, and transmits a DMRS (2C) at Port-1 and transmits a DMRS (1D)at Port-2. In addition, the BS may transmit information indicating DMRSs1C, 2C and 1D to the UE according to any one of the embodiments 1-1 to3. The UE can detect DMRSs 1C, 2C and 1D respectively transmitted atPort-0, Port-1, and Port-2 on the basis of the above-mentionedindication information, and can demodulate three layers using thedetected DMRSs. In addition, the UE may recover one codeword from alayer transmitted at Port-0 from among the three layers, and may alsorecover another codeword transmitted either from a layer transmitted atPort-1 or from a layer transmitted at Port-2. Therefore, the UE canobtain two codewords transmitted from the BS to the UE.

5. The Mapping of a Codeword to a DMRS Port as Shown in Table 6 or Table8

Referring to Table 6 or Table 8, the BS can transmit one layercorresponding to CW-0 through Port-0, and can transmit two layerscorresponding to CW-1 through Port-2 and Port-3. Assuming that atransmission DMRS for each port is determined according to Table 4, theBS transmits a DMRS (1C) at Port-0, transmits a DMRS (1D) at Port-2, andtransmits a DMRS (2D) at Port-3. The BS can transmit informationindicating DMRSs 1C, 1D and 2D to the UE according to any one of theabove-mentioned embodiments 1-1 to 3. The UE can detect DMRS 1C, DMRS1D, and DMRS 2D respectively transmitted at Port-0, Port-2, and Port-3on the basis of the above-mentioned indication information, and candemodulate three layers using the detected DMRSs. In addition, the UEcan recover one codeword from a layer transmitted at Port-0 from amongthe three layers, and can recover a layer transmitted at Port-2 andanother codeword from a layer transmitted at Port-3.

Embodiment 4 or Embodiment 5 is combined with any one of Embodiments 1-1to 3 so as to be implemented by the BS and the UE.

The BS processor 400 b configures one or more codewords using one ormore layers, controls the BS transmitter 100 b such that each layer canbe transmitted through the corresponding DMRS port according toEmbodiment 4 or Embodiment 5. In addition, the BS processor 400 b canspread one or more DMRSs for one or more layers according to any one ofthe embodiments 1-1 to 3, and can allocate the spread DMRS(s) to one ortwo CDM groups. Under the control of the BS processor 400 b, the BStransmitter 100 b may transmit the one or more DMRSs along with the oneor more layers at the corresponding antenna port.

The UE processor 100 a can detect DMRS(s) allocated to the UE accordingto any one of the embodiments 1-1 to 3, and can detect/receive thecorresponding layer(s) using the detected DMRS(s). Under the control ofthe UE processor 100 a, the UE receiver 300 a detects the DMRS(s)allocated to the UE, and can detect/receive the corresponding layer(s)using the detected DMRS(s). The UE processor 100 a may recover thedetected layer(s) using one or more codewords according to Embodiment 4or Embodiment 5.

<Downlink Control Signaling for Dynamic Switching>

The above-mentioned embodiments have mainly disclosed that a DMRS portfor each rank is predefined. For example, according to Embodiment 4,DMRS Port-0 is used for Rank-1 transmission, and DMRS Port-0, DMRSPort-1, and DMRS Port-2 are used for Rank-3 transmission. According toEmbodiment 5, DMRS Port-0 is used for Rank-1 transmission, and DMRSPort-0, DMRS Port-2, and DMRS Port-3 are used for Rank-3 transmission.As can be seen from Tables 5 to 8, during Rank-1 transmission, DMRS istransmitted only through Port-0 and a single codeword is transmittedonly to Rank-1.

In accordance with the legacy standard, during transmission of a singlecodeword, the corresponding data is transmitted only to Rank-1 throughPort-0 as shown in Tables 5 to 8. For example, under the condition thatthe BS desires to transmit a TB (hereinafter referred to as TB1) to aUE1 of the SU-MIMO mode and desires to transmit a TB (hereinafterreferred to as TB2) to a UE2 of the SU-MIMO mode, the BS performs Rank-1transmission of a codeword of TB1 to the UE1 through DMRS Port-0according to the legacy standard, and performs Rank-1 transmission of acodeword of TB2 to the UE2 through DMRS Port-0 at another time resource.If the BS desires to simultaneously transmit a TB1 and a TB2 to a UE1and a UE2 in a MU-MIMO mode, the UE1 and the UE2 must reconfigure theirsystem in the MU-MIMO mode, such that the UE1 and the UE2 can detecttheir own TBs from among TB1 and TB2. However, semi-static switchingcapable of reconfiguring the system by higher layer signaling requires arelatively longer time for mode switching as compared to dynamicswitching by PDCCH or the like. Therefore, various embodiments forswitching a dynamic mode between an SU-MIMO and an MU-MIMO willhereinafter be described in detail.

In order to switch transmission modes between SU-MIMO and MU-MIMO bydynamic switching, it is necessary to indicate another DMRS port at thesame rank.

Various embodiments regarding a method for indicating a DMRS port at apredetermined rank in such a manner that a port for DMRS transmission ischanged at the predetermined rank will hereinafter be described indetail. In addition, other embodiments regarding a method for indicatinga DMRS port at single codeword/TB transmission caused by multi-layertransmission will also be described. Although the embodiments of thepresent invention will hereinafter be described using transmission of amaximum of 2 codewords as an example, it should be noted that thefollowing embodiments can also be applied to the other case in which twoor more codewords are transmitted.

By any DCI format (for example, Format 2, Format 2A, Format 2B, orFormat 2C) related to spatial multiplexing, a deactivated TB (alsocalled a disabled TB) from among multiple codewords may be indicated.The BS according to the embodiments of the present invention indicates aDMRS port using a New Data Indicator (NDI) for each TB, resulting inMU-MIMO transmission. In DCI Format 2, 2A, 2B or 2C, each TB may includean MCS field, an NDI field, a Redundancy Version (RV) field, etc. TheMCS field indicates an MCS level of the corresponding TB, and the RVfield indicates ‘rv_(idx)’ of the corresponding TB. The conventional NDIfield is used to indicate an antenna port for single antenna porttransmission (one TB disable). In more detail, if an NDI of the disabledTB is set to 0, Antenna Port #7 is used for single antenna porttransmission. If an NDI of the disabled TB is set to 1, Antenna Port #8is used for single antenna port transmission.

Meanwhile, a TB may be disabled according to values established in DCIformats 2, 2A, 2B, and 2C. For example, in DCI Formats 2, 2A, 2B, 2C, ifan MCS level indicated in an MCS field corresponding to a predeterminedTB is denoted by I_(MCS)=0 and an RV indicated by the RV field isdenoted by rv_(idx)=1, the predetermined TB is enabled. The disabled TBis not transmitted, and only the enabled TB is transmitted.

Table 9 shows one example for supporting DMRS port indication either atRank-1 transmission or at single TB transmission caused by multiplelayers.

TABLE 9 TBs and NDI Rank indication Indicated DMRS index set One TB isdisabled 1 Port-0 (NDI = 0 in disabled TB) One TB is disabled Port-1(NDI = 1 in disabled TB) No disabled TBs 2 Port-0, Port-1 One TB isdisabled 2 Port-0, Port-1 No disabled TBs 3 Port-0, Port-1, Port-2 OneTB is disabled 3 Port-0, Port-1, Port-2 No disabled TBs 4 Port-0,Port-1, Port-2, Port-3 One TB is disabled 4 Port-0, Port-1, Port-2,Port-3 No disabled TBs 5 Port-0, Port-1, Port-2, Port-3, Port-4 Nodisabled TBs 6 Port-0, Port-1, Port-2, Port-3, Port-4, Port-5 Nodisabled TBs 7 Port-0, Port-1, Port-2, Port-3, Port-4, Port-5, Port-6 Nodisabled TBs 8 Port-0, Port-1, Port-2, Port-3, Port-4, Port-5, Port-6,Port-7

In Table 9, single codeword transmission disables one TB from among twoTBs using a DCI such that it can be signaled to a UE. Particularly, anNDI exemplarily shows that a DMRS port is indicated only in Rank-1transmission. Referring to Table 9, the BS may transmit a layer of TB1to a UE1 through DMRS Port-0 as an example. In this case, the BS mayconfigure downlink control information of the UE1 in such a manner thatdisablement of TB2 is represented by such downlink control information.In addition, the BS may transmit downlink control information in whichan NDI for the disabled TB2 is set to 0. The UE1 detects downlinkcontrol information of the UE1 using blind decoding. Based on the NDIassigned to 0, the UE1 can recognize that a layer of a disabled TB1 hasbeen transmitted through DMRS Port-0. The UE1 detects a DMRS transmittedat DMRS Port-0, and can recover a layer transmitted for the UE1 into theTB1 using the detected DMRS. In addition, the BS may transmit a layer ofthe TB2 to a UE2 through DMRS Port-0. The BS configures downlink controlinformation of the UE2 so as to indicate disablement of TB1, andtransmits the configured information to the UE2. In addition, the BSconfigures downlink control information of the UE2 in such a manner thatan NDI for the TB1 is set to 0, such that resultant UE2 downlink controlinformation can be transmitted to the UE2. The UE2 detects the UE2downlink control information by blind decoding, and can recognize that alayer of an enabled TB2 has been transmitted through DMRS Port-1 on thebasis of NDI assigned to 1 (NDI=1). The BS may perform SU-MIMOtransmission in which TB1 and TB2 are transmitted through different timezones. In the same time zone, the BS may also perform MU-MIMOtransmission in which TB1 is transmitted at DMRS Port-0 and TB2 istransmitted at DMRS Port-1. From the viewpoint of UE1 and UE2, it can berecognized that the BS has transmitted a single codeword on the basis ofthe corresponding downlink control information, such that the UE1 andthe UE2 can detect the corresponding layer without changing the systemparameter from SU-MIMO to MU-MIMO. The above-mentioned single codewordtransmission for each UE may be used only in retransmission and/orinitial transmission.

Table 9 exemplarily shows that a DMRS port is indicated only in Rank-1transmission. However, even in the case of single codeword transmissioncaused by multiple layers, used DMRS ports are changed, and the usedDMRS port set may be indicated by NDI setting for a disabled TB. Table10 shows another example for supporting DMRS port indication either atRank-1 transmission or at single TB transmission caused by multiplelayers.

TABLE 10 TBs and NDI Rank indication Indicated DMRS index set One TB isdisabled 1 Port-0 (NDI = 0 in disabled TB) One TB is disabled Port-1(NDI = 1 in disabled TB) No disabled TBs 2 Port-0, Port-1 One TB isdisabled 2 Port-0, Port-1 (NDI = 0 in disabled TB) One TB is disabled 2{Port-0, Port-1} or {Port-4, (NDI = 1 in disabled TB) Port-6} orreserved No disabled TBs 3 Port-0, Port-1, Port-2 One TB is disabled 3Port-0, Port-1, Port-2 (NDI = 0 in disabled TB) One TB is disabled 3Port-0, Port-1, Port-4 (NDI = 1 in disabled TB) No disabled TBs 4Port-0, Port-1, Port-2, Port-3 One TB is disabled 4 Port-0, Port-1,Port-2, Port-3 (NDI = 0 in disabled TB) One TB is disabled 4 Port-0,Port-1, Port-4, Port-6 (NDI = 1 in disabled TB) No disabled TBs 5Port-0, Port-1, Port-2, Port-3, Port-4 No disabled TBs 6 Port-0, Port-1,Port-2, Port-3, Port-4, Port-5 No disabled TBs 7 Port-0, Port-1, Port-2,Port-3, Port-4, Port-5, Port-6 No disabled TBs 8 Port-0, Port-1, Port-2,Port-3, Port-4, Port-5, Port-6, Port-7

As can be seen from Table 10, DMRS port(s) are allocated to layer(s),such that one or two CDM groups may be used for multi-layer singlecodeword transmission. For example, referring to Table 10, in the caseof Rank-3 transmission, three layers constructing two TBs may betransmitted through Port-0, Port-1, and Port-2, or three layersconstructing one TB may be transmitted through Port-0, Port-1, andPort-2, and three layers constructing one TB may be transmitted throughPort-0, Port-1, and Port-4. The BS disables one TB, sets an NDI for thedisabled TB to zero 0, and transmits the resultant data, such that itcan be recognized that the three layers are transmitted through Port-0,Port-1, and Port-2. In addition, the BS disables one TB, sets an NDI forthe disabled TB to ‘1’, and transmits the resultant data, such that itcan be recognized that the three layers are transmitted through Port-0,Port-1, and Port-2.

In accordance with the embodiments for indicating a DMRS port using anNDI, different information for indicating a DMRS port need not beadditionally transmitted, resulting in reduction in downlink signalingoverhead.

The above-mentioned embodiment in which DMRS port(s) used for singlecodeword transmission are changed using an NDI and the DMRS port(s) areindicated may be combined with the embodiments 1-1 to 5 as necessary.For example, not only an OCC used when each DMRS port spreads thecorresponding DMRS, but also a CDM group for transmitting thecorresponding DMRS may be determined according to any one of theembodiments 1-1 to 3.

The BS processor 400 b according to the present invention enables onlyone available TB, and disables the remaining TB(s), such that it canallocate the resultant TBs to a UE. The BS processor 400 b may informthe UE of disablement of the remaining TB(s) using downlink controlinformation. For example, the BS processor 400 b sets a predeterminedvalue to an MCS and RV of the remaining TB(s) contained in downlinkcontrol information, such that the remaining TB(s) can be disabled. TheBS processor 400 b configures a codeword corresponding to one TB whichis enabled, and controls the BS transmitter 100 b, such that the BStransmitter 100 b can transmit the codeword using one or more layers.

The BS processor 400 b allocates an antenna port to each layer of thecodeword, and controls the BS transmitter 100 b in such a manner thatthe corresponding layer and DMRS can be transmitted at the allocatedantenna port. The BS processor 400 b sets a predetermined value to anNDI for the disabled TB, such that it can inform the UE of an antennaport set through which layer(s) of the codeword is transmitted.Referring to Table 10, if the UE feeds back Rank-3 transmission to theBS as rank information, the BS processor 400 b sets the value of 1 to anNDI of the disabled TB, such that it is possible to perform signaling ofinformation indicating that three layers allocated to the UE aretransmitted through Port-0, Port-1 and Port-2.

Under the control of the BS processor 400 b, the BS transmitter 100 btransmits the downlink control information within a control region, andeach layer of the codeword can be transmitted along with thecorresponding DMRS through the allocated antenna port.

The receiver 300 a of the UE receives the downlink control information,and transfers the received information to the UE processor 400 a. Uponreceiving the downlink control information, the UE processor 400 a canrecognize disablement of the remaining TBs other than one TB. That is,the UE processor 400 a can recognize that a single codeword istransmitted to the UE on the basis of the control information. Inaddition, the UE processor 400 a can recognize, on the basis of an NDIfor each TB of the downlink control information, an antenna port setthrough which layer(s) for single codeword transmission are transmitted.The UE processor 400 a can control the UE receiver 300 a in such amanner that the corresponding DMRS can be detected from each antennaport of the antenna port set on the basis of the downlink controlinformation. In addition, the UE processor 400 a can control the UEreceiver 300 a in such a manner that the transmitted layer(s) can bedetected and received on the basis of the detected DMRS(s). The UEprocessor 400 a can recover the received layer(s) into the codeword.

It will be apparent to those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

INDUSTRIAL APPLICABILITY

The embodiments of the present invention can be applied to a BS, a UE,or other communication devices in a wireless communication system.

The invention claimed is:
 1. A method for transmitting downlink data toa user equipment (UE) in a wireless communication system, the methodcomprising: transmitting rank information for indicating a number oftransmission layers for the downlink data to the UE; and transmittingdemodulation reference signals (DMRSs) of the transmission layers forthe downlink data for demodulation of the downlink data through a firstcode division multiplexing (CDM) group and a second (CDM) group to theUE, wherein the first CDM group is configured with a predetermined firsttime-frequency resource set and the second CDM group is configured witha predetermined second time-frequency resource separate from the firsttime-frequency resource set, wherein each of the DMRSs of thetransmission layers for the downlink data is transmitted in the firstCDM group or the second CDM group using one of predetermined orthogonalcover codes (OCCs) according to a predefined mapping rule, and whereinthe predefined mapping rule defines a one-to-one correspondence between8 DMRSs and 8 transmission layers, and maps each of 8 DMRSs for 8transmission layers to the first and second CDM groups and to one of thepredetermined OCCs.
 2. The method according to claim 1, wherein thepredefined mapping rule maps DMRSs for layers 0, 1, 4 and 6 of the 8layers to the first CDM group and DMRSs for layers 2, 3, 5 and 7 of the8 layers to the second CDM group.
 3. The method according to claim 1,wherein the predetermined OCCs have a length-4.
 4. The method accordingto claim 1, wherein the predefined mapping rule maps each of DMRSsmapped to a same CDM group to different one of OCCs available to thesame CDM group.
 5. The method according to claim 1, further comprising:transmitting mapping type information indicating the predefined mappingrule among a plurality of different predetermined mapping rules.
 6. Amethod for receiving downlink data by a user equipment (UE) in awireless communication system, the method comprising: receiving, by theUE, rank information for indicating a number of transmission layers forthe downlink data; receiving, by the UE, demodulation reference signals(DMRSs) of the transmission layers for the downlink data through a firstcode division multiplexing (CDM) group and a second CDM group fordemodulating the downlink data based on the rank information, whereinthe first CDM group is configured with a predetermined firsttime-frequency resource set and the second CDM group is configured witha predetermined second time-frequency resource separate from the firsttime-frequency resource set, wherein each of the DMRSs of thetransmission layers for the downlink data is received in the first CDMgroup or the second CDM group using one of predetermined orthogonalcover codes (OCCs) according to a predefined mapping rule, and whereinthe predefined mapping rule defines a one-to-one correspondence between8 DMRSs and 8 transmission layers, and maps each of 8 DMRSs for 8transmission layers to the first and second CDM groups and to one of thepredetermined OCCs.
 7. The method according to claim 6, wherein thepredefined mapping rule maps DMRSs for layers 0, 1, 4 and 6 of the 8layers to the first CDM group and DMRSs for layers 2, 3, 5 and 7 of the8 layers to the second CDM group.
 8. The method according to claim 6,wherein the predetermined OCCs have a length-4.
 9. The method accordingto claim 6, wherein the predefined mapping rule maps each of DMRSsmapped to a same CDM group to different one of OCCs available to thesame CDM group.
 10. The method according to claim 6, further comprising:receiving mapping type information indicating the predefined mappingrule among a plurality of different predetermined mapping rules.
 11. Abase station (BS) for transmitting downlink data to a user equipment(UE) in a wireless communication system, the BS comprising: atransmitter configured to transmit the downlink data to the UE; and aprocessor configured to control the transmitter: to transmit rankinformation for indicating a number of transmission layers for thedownlink data to the UE, and to transmit demodulation reference signals(DMRSs) of the transmission layers for the downlink data fordemodulation of the downlink data through a first code divisionmultiplexing (CDM) group and a second (CDM) group to the UE, wherein thefirst CDM group is configured with a predetermined first time-frequencyresource set and the second CDM group is configured with a predeterminedsecond time-frequency resource separate from the first time-frequencyresource set, wherein each of the DMRSs of the transmission layers forthe downlink data is transmitted in the first CDM group or the secondCDM group using one of predetermined orthogonal cover codes (OCCs)according to a predefined mapping rule, and wherein the predefinedmapping rule defines a one-to-one correspondence between 8 DMRSs and 8transmission layers, and maps each of 8 DMRSs for 8 transmission layersto the first and second CDM groups and to one of the predetermined OCCs.12. The BS according to claim 11, wherein the predefined mapping rulemaps DMRSs for layers 0, 1, 4 and 6 of the 8 layers to the first CDMgroup and DMRSs for layers 2, 3, 5 and 7 of the 8 layers to the secondCDM group.
 13. The BS according to claim 11, wherein the predeterminedOCCs have a length-4.
 14. The BS according to claim 11, wherein thepredefined mapping rule maps each of DMRSs mapped to a same CDM group todifferent one of OCCs available to the same CDM group.
 15. The BSaccording to claim 11, wherein the processor is configured to controlthe transmitter to transmit mapping type information indicating thepredefined mapping rule among a plurality of different predeterminedmapping rules.
 16. A user equipment (UE) for receiving downlink data ina wireless communication system, the UE comprising: a receiver; and aprocessor configured to control the receiver: to receive rankinformation for indicating a number of transmission layers for thedownlink data, and to receive demodulation reference signals (DMRSs) ofthe transmission layers for the downlink data through a first codedivision multiplexing (CDM) group and a second CDM group fordemodulating the downlink data based on the rank information, whereinthe first CDM group is configured with a predetermined firsttime-frequency resource set and the second CDM group is configured witha predetermined second time-frequency resource separate from the firsttime-frequency resource set, wherein each of the DMRSs of thetransmission layers for the downlink data is received in the first CDMgroup or the second CDM group using one of predetermined orthogonalcover codes (OCCs) according to a predefined mapping rule, and whereinthe predefined mapping rule defines a one-to-one correspondence between8 DMRSs and 8 transmission layers, and maps each of 8 DMRSs for 8transmission layers to the first and second CDM groups and to one of thepredetermined OCCs.
 17. The UE according to claim 16, wherein thepredefined mapping rule maps DMRSs for layers 0, 1, 4 and 6 of the 8layers to the first CDM group and DMRSs for layers 2, 3, 5 and 7 of the8 layers to the second CDM group.
 18. The UE according to claim 16,wherein the predetermined OCCs have a length-4.
 19. The UE according toclaim 16, wherein the predefined mapping rule maps each of DMRSs mappedto a same CDM group to different one of OCCs available to the same CDMgroup.
 20. The UE according to claim 16, wherein the processor isconfigured to control the transmitter to transmit mapping typeinformation indicating the predefined mapping rule among a plurality ofdifferent predetermined mapping rules.